Hitting the Wall in a Marathon: What It Is, Why It Happens, How to Avoid It (UK 2026 Guide)

Every first-time marathoner has heard the phrase. It comes up in podcasts, in pub conversations with the runner at the next table, in the nervous chat at the start line. "Just watch out for the wall." It sounds mystical, like something that happens to other people, like a punishment for runners who did something wrong. It is not mystical at all. The wall is a measurable biological event that can be predicted with a calculator, prepared for with a plan, and in most cases avoided entirely.

When you read about elite marathoners crashing in the last 6 miles, when you hear race reports about runners walking at mile 22 with their hands on their knees, when you see those photos of the Boston finish chute full of people shuffling and weeping, you are looking at glycogen depletion. The wall is what your body does when its preferred fuel runs out and it has to switch, mid-effort, to a backup system that is slower, less efficient, and harder on your legs and brain.

For UK beginners training for London, Manchester, Edinburgh, Brighton, or any other marathon in 2026, understanding the wall is not optional. It is the single biggest variable between a marathon you remember as proud and a marathon you remember as suffering. The good news is that the four things that prevent the wall are entirely within your control. The better news is that the runners who suffer most are usually the ones who never learned what was happening to their body in the first place.

This guide explains what the wall actually is at a biological level, why beginners hit it earlier than experienced runners, and the four-step prevention plan that works for almost everyone. There is also an interactive Wall Predictor below that you can use to estimate where your own fuel tank will run dry based on your weight, pace, training, and race-day fuelling plan. Use it honestly.

What is "the wall" in marathon running

The wall, sometimes called "bonking" by cyclists and "hitting empty" by ultrarunners, is the moment in a long endurance effort when your body's stored carbohydrate, called glycogen, runs out. Glycogen is stored in your muscles and your liver, and it is the fuel your body prefers to burn when you are running at marathon pace or faster. When the supply is exhausted, your body is forced to rely on a slower-burning fuel, body fat, which cannot deliver energy fast enough to maintain your race pace. The result is a dramatic slowdown, often by 2 to 4 minutes per mile, accompanied by heavy legs, mental fog, dizziness, nausea, and in some cases temporary tunnel vision.

What makes the wall feel so brutal is the speed of the transition. One mile you are running comfortably at your goal pace. The next mile, your legs feel like they are wading through wet cement and your brain is struggling to remember what direction the finish line is in. It is not gradual fatigue. It is a fuel-system switch that happens within a few hundred metres, and once it has started, you cannot reverse it mid-race. You can only manage it.

The wall typically appears between mile 18 and mile 22 for runners who have not fuelled correctly. For elite athletes who have trained their fat-burning systems extensively and consume carbohydrate aggressively during the race, the wall can be pushed past mile 26 entirely, which is why you rarely see top-tier marathoners walking in the final stretch. For first-time marathoners who started too fast and skipped their gels, the wall can arrive as early as mile 16. The exact mile varies, but the mechanism is identical for everyone: stored carbohydrate has run out.

The science of glycogen depletion

How your body stores carbohydrate

Every gram of carbohydrate you eat is broken down into glucose, the simple sugar that fuels your muscles and brain. What you do not burn immediately gets stored as glycogen, which is essentially a tightly packed chain of glucose molecules. Your body keeps glycogen in two places: roughly 100 grams in your liver, where it stabilises blood sugar between meals, and roughly 400 to 500 grams in your skeletal muscles, where it powers movement. Together that adds up to about 2,000 calories of carbohydrate stored in a typical runner's body. The exact number scales with body weight and training history, but 2,000 is a useful round number for marathon planning.

Importantly, you cannot top up your glycogen stores indefinitely. Your muscles and liver have a ceiling. This is why carb-loading is a 2 to 3 day process rather than a single huge pasta dinner. The body needs time to absorb and store the extra glucose. A trained runner who has carb-loaded properly can push their glycogen ceiling roughly 50 percent higher than baseline, taking them from around 2,000 calories of stored fuel to around 3,000. That extra 1,000 calories is the difference between hitting the wall at mile 19 and finishing the marathon strong.

Why running burns glycogen first

Your body has two main fuel systems: glycogen (carbohydrate) and body fat. Both are stored in large quantities, even in lean runners. The difference is the rate at which they can be turned into usable energy. Glycogen burns fast and clean, producing a lot of ATP (the molecule your muscles use as their immediate energy currency) per second. Fat burns slowly, requires oxygen, and cannot match the energy output that hard running demands. When you are running easy, your body burns mostly fat and spares glycogen. When you are running at marathon pace, the mix shifts heavily toward glycogen because fat alone cannot keep up with the energy demand of that pace.

This is why pace discipline matters so much in a marathon. Every minute you spend running faster than your true sustainable pace, you are burning a higher proportion of glycogen and a lower proportion of fat. You are accelerating the rate at which your fuel tank empties. The runners who go out at 8:30 minute miles when their true pace is 9:30 might feel great at mile 10, but they are guaranteeing themselves a wall encounter somewhere around mile 18. Glycogen is a finite resource, and pace is the throttle that controls how fast you drain it.

What happens when it runs out

When your muscle glycogen is depleted, two things happen almost simultaneously. First, your muscles can no longer produce force at the same rate, because fat cannot deliver energy fast enough to maintain your race pace. Your legs feel suddenly heavy, almost like someone has filled your shoes with sand. Pace drops dramatically even when you try to push harder. Second, your liver glycogen, which has been quietly keeping your blood sugar stable, also starts to run low. This affects your brain, which depends on glucose for fuel. The result is mental fog, slowed thinking, sometimes mild dizziness, and a creeping sense of confusion or low mood. Runners often describe the wall as feeling like all the joy has been drained from the race in the space of one mile.

Some runners experience nausea, tunnel vision, or shakiness, especially if they have also under-hydrated. In extreme cases, runners experience temporary loss of coordination, slurred speech, or what feels like a dissociative state. None of this is dangerous in the medical sense, but it is extremely unpleasant, and it is the reason so many first-time marathoners describe the last 6 miles as the hardest thing they have ever done. The good news is that all of this is preventable.

Why most beginners hit the wall

1. Pacing too fast in the first 13 miles

This is the single most common cause. Adrenaline, crowd energy, fresh legs, and the addictive feeling of running easy in the early miles tempt almost every beginner into running 30 to 60 seconds per mile faster than their training pace. The faster you run, the higher the percentage of glycogen in your fuel mix. By the time you reach the halfway point, you have already burned through far more glycogen than your race plan budgeted for, and the wall arrives early. The fix is to deliberately run the first 10K slower than you think you should, then hold steady through mile 20 and only push the pace in the final 10K if you have fuel left.

2. Not carb-loading

Many beginners treat carb-loading as a single big pasta dinner the night before. That is not carb-loading, that is just dinner. Real carb-loading is a 2 to 3 day process of eating roughly 8 to 10 grams of carbohydrate per kilogram of body weight per day, which for a 70 kg runner means around 560 to 700 grams of carbs daily. This is a lot more food than most people are used to, and it requires deliberate planning. Done properly, it can lift your glycogen stores by around 50 percent, adding roughly 1,000 calories of fuel to your tank, which is the equivalent of 10 extra miles.

3. Skipping in-race gels

A single energy gel provides roughly 100 calories of fast-absorbing carbohydrate. Taking a gel every 30 minutes from mile 4 onward gives your body a steady stream of glucose that supplements your glycogen stores and delays depletion. Many beginners skip gels because they feel fine in the early miles or worry about stomach upset. By the time they realise they need fuel, it is too late, because your stomach largely shuts down absorption once you have started bonking. The rule is simple: start fuelling before you need it, and keep fuelling every 30 minutes regardless of how you feel.

4. Not training enough long slow runs

Long slow runs are the single most important type of training for marathon preparation, and they work specifically by improving your body's ability to burn fat at running pace. A runner who has done six or seven 18 to 20 mile long runs in the months before race day has trained their mitochondria, capillaries, and enzyme systems to extract more energy from fat, which spares glycogen for the moments when you really need it. A runner who has skipped the long runs because they felt boring or tiring will burn glycogen faster mile-for-mile and hit the wall sooner. The body adapts to what you train it to do.

5. Race day too warm or dehydration accelerates glycogen burn

When you are dehydrated, your body has to work harder to cool itself, your blood volume drops, and your heart rate climbs at any given pace. All of this increases your overall energy demand and shifts your fuel mix further toward glycogen. A hot race day can deplete your stores 10 to 20 percent faster than a cool day at the same pace. This is why UK runners who train through a cool spring and then race in an unexpectedly warm 18-degree London afternoon often hit the wall harder than expected. Hydrate aggressively in the days before, take water at every aid station, and slow your goal pace if conditions are warmer than you trained in.

The 4-step plan to avoid the wall

1. Long slow runs to train fat burning

Build your training plan around a weekly long slow run that progressively increases from 8 miles in week one to 18 or 20 miles in your peak weeks. Run these at a pace where you can hold a conversation, which for most beginners is around 90 to 120 seconds per mile slower than goal marathon pace. The point is not to practice race pace, the point is to spend time on your feet teaching your body to burn fat efficiently. This is the foundation everything else is built on.

2. Carb-load 2 to 3 days before

Starting two days before race day, shift your meals toward easily digested carbohydrate sources: rice, pasta, bread, potatoes, oats, fruit, sports drinks. Aim for roughly 8 to 10 grams of carbohydrate per kilogram of body weight per day. Avoid trying anything new or experimental, and keep fibre moderate so your digestive system is not surprised on race morning. Stop the heavy carb-loading dinner at lunchtime the day before, then have a smaller carb-focused dinner that evening to avoid a heavy stomach on race day.

3. Fuel every 30 minutes in the race

Take your first gel at mile 4, not mile 10. Take another every 30 minutes after that. For a 4-hour marathon, that means 7 to 8 gels across the race. Wash each one down with water at the next aid station. If gels cause stomach problems for you, try chews, jelly babies, or a sports drink, but practice your fuelling in training so that race day is not the first time you have eaten while running. Aim for roughly 60 grams of carbohydrate per hour, which is roughly two gels per hour.

4. Pace discipline, first 10K at easy effort

Your first 10K should feel almost embarrassingly easy. You should be holding back, letting other runners pass you, focused on running 10 to 15 seconds per mile slower than your goal pace. From 10K to 30K, settle into goal pace. From 30K to the finish, give whatever you have left. This negative-split approach burns glycogen at a sustainable rate early in the race when you cannot feel the consequences yet, and rewards you with fuel still in the tank when you actually need to push.

"The wall isn't bad luck. It is what happens when your body's fuel tank empties faster than your race plan accounted for."

What to do if you DO hit the wall mid-race

Even with perfect preparation, sometimes the wall still arrives. Maybe the weather changed, maybe your pacing slipped, maybe a stomach issue meant a gel did not get absorbed. If you feel the wall coming on, usually a sudden onset of heavy legs and mental fog somewhere after mile 18, the first thing to do is slow down immediately. Trying to power through at the same pace will make the next mile worse, not better. Drop your pace by 60 to 90 seconds per mile and accept that your finish time is changing.

The second thing to do is eat. Take a gel, then take another within 10 minutes. Wash both down with water. Some runners carry a sachet of sugar or a small bag of jelly babies precisely for this moment. Walk through the next aid station while you sip a sports drink and let the sugar absorb. You will not feel an instant recovery, glycogen does not refill that fast mid-race, but the incoming glucose will steady your blood sugar and prevent further mental decline. Twenty minutes of slower running and aggressive fuelling is the difference between finishing in 4:45 and being pulled off the course at mile 22.

The third thing is mental. Stop thinking about the finish line, stop thinking about your goal time, and shrink your focus to the next mile only. The next lamppost. The next aid station. The next minute. Many runners who hit the wall and still finish describe it as a series of tiny goals strung together: get to the next aid station, walk for one minute, run for five, repeat. The clock will be slower than you wanted, but you will cross the line, and the lesson you take away from that race will make every future marathon better.

How Edge fits marathon training

Edge is a UK fitness app with over 17,000 members, built around adaptive starting plans that match your current fitness, your weekly availability, and your goal event. When you tell Edge you are training for a marathon, your starting plan is built around progressively longer long slow runs, the single most important type of training for wall prevention, alongside easier midweek runs, strength sessions, and mobility work. The long runs are scheduled so they build gradually without overloading your body, because that is how your fat-burning system actually adapts.

Edge includes coach video demos for every strength and mobility move, progress tracking, and syncing with Strava, Garmin, Apple Watch, and Coros so your runs and workouts flow into one place. The Flexi Swap feature means if life gets in the way mid-week, you can shuffle sessions around without breaking the plan. Edge AI is available when you ask it a question, giving you a 30-second answer on training, technique, or recovery, drawn from the same knowledge base our coaches use. You are not on your own, but you also are not micromanaged.

What Edge will not do for you is carb-load, take your gels, or hold back your pace in the first 10K. Those are still on you. What Edge does do is make sure the foundation, your weekly long run progression and your overall fitness, is solid enough that the four-step plan in this guide can actually work on race day. You can try Edge free for 7 days, then it is £19.99 monthly or £119.99 annually.

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