Training
Every marathon runner has heard the phrase. Many have experienced it. Very few understand exactly what is happening in their body when it happens, which is why they keep making the same mistakes. Here is the full picture, and what to do about it.
What Is Actually Happening
Hitting the wall has a precise physiological explanation. Your body stores carbohydrate as glycogen, primarily in your liver and muscles. At marathon race pace, glycogen is your dominant fuel source. The problem is that your total glycogen stores are limited, typically equivalent to around 90 to 120 minutes of high-intensity running. When those stores run out, the body cannot immediately replace them mid-race, and your ability to sustain marathon pace collapses.
The result is not a gradual slowdown. It is an abrupt, often brutal deterioration. Legs that felt manageable at mile 17 feel like concrete at mile 20. The brain, which is almost entirely glucose-dependent, also suffers: decision-making deteriorates, motivation collapses, and a creeping sense of despair sets in that feels completely disproportionate to the situation.
This is not weakness. It is biochemistry. And because it is biochemistry, it is predictable and, more importantly, largely preventable.
"Hitting the wall is not something that happens to unlucky runners. It is something that happens to runners who either went out too fast, under-fuelled, or both."
The Key Variables
The single biggest predictor of hitting the wall is going out too fast. At marathon pace, your body is burning a mix of glycogen and fat. When you run faster than marathon pace, glycogen usage spikes dramatically. A runner who covers the first 10 miles at 5 to 10 seconds per mile faster than their target pace will exhaust their glycogen stores significantly earlier than a runner who goes through even splits.
The excitement of race day, the crowd, the adrenaline, and the slightly downhill start of many major marathons all conspire to make you run too fast in the early miles. This is the most common cause of walls in runners who have trained adequately.
Even with conservative pacing, glycogen will deplete over a marathon. Taking on carbohydrates during the race, via gels, chews, or sports drinks, delays that depletion by maintaining blood glucose levels and providing an external carbohydrate source. Runners who do not fuel, or who start fuelling too late (after mile 13 is already too late), are significantly more likely to hit the wall regardless of how well they trained.
The evidence-based guideline is to take 30 to 60 grams of carbohydrate per hour from around 30 to 40 minutes into the race, at regular intervals of every 20 to 25 minutes. Waiting until you feel you need it is too late: by the time you feel the effects of glycogen depletion, the process is already well advanced.
Well-trained runners hit the wall less often because years of training have improved their ability to burn fat at marathon pace. Fat oxidation at race pace supplements glycogen and extends how long stores last. This adaptation is built through high mileage over many months, particularly easy aerobic running. It cannot be rushed, and it cannot be replicated by a 12-week training plan.
The Prevention Plan
Maximising glycogen stores before the race gives you more fuel to start with, which delays depletion. Effective carbohydrate loading means consuming 7 to 10 grams of carbohydrate per kilogram of body weight per day in the two to three days before the race, while tapering training intensity significantly. This is not an excuse to eat anything. The focus is on easily digestible carbohydrates: rice, pasta, bread, potatoes, and sports drinks. Fat, fibre, and novel foods should be minimised to reduce the risk of digestive issues on race day.
The gut can be trained. Runners who practise taking gels during long training runs develop better gut absorption of carbohydrates during exercise, and are much less likely to experience nausea or rejection on race day. Every long run of 16 miles or more should include fuelling practice at race-day timing and quantities. Turning up to the marathon having never taken a gel on the run is an avoidable risk.
Set a pace target based on your training, stick to it in the first half, and resist the temptation to bank time early. Many runners who feel strong at mile 10 run significantly faster than planned, only to pay for it after mile 20. A pace that feels almost embarrassingly easy in the first 10 miles is usually the correct pace. The goal is to cross halfway feeling like you have only just started to work.
Long runs done at a proper easy pace build the aerobic and fat-burning adaptations that reduce glycogen dependency at race pace. Runners who do their long runs too fast miss this adaptation and arrive at the race with a higher glycogen demand than their fitness actually requires.
Mid-Race Management
If the wall arrives despite your best preparation, your goal shifts immediately from hitting a time to reaching the finish line. Slow down to a pace your body can sustain. Walking, while it feels like defeat in the moment, is a legitimate strategy that reduces glycogen demand and allows fat burning to partially compensate.
Take on carbohydrates immediately and repeatedly. Even at this stage, maintaining blood glucose will help your brain and slow the deterioration. If the course offers gels or sports drinks at aid stations, take them at every opportunity.
The mental challenge of hitting the wall is as significant as the physical one. The despair that accompanies glycogen depletion is partly neurological, not a reliable signal that something is seriously wrong. Remind yourself that the brain is exaggerating. Slow, steady forward movement will get you to the finish. The worst feeling will not last.
A debrief with your coach after a wall is invaluable. It is almost always possible to identify exactly where the race went wrong, whether in training, pacing, or fuelling, and to correct it for next time. Hitting the wall once, understood properly, usually means never hitting it again.
Common Questions
Most runners hit the wall between miles 18 and 22, corresponding to roughly 2 to 3 hours of running at marathon pace. This is when glycogen stores are typically depleted if fuelling has been insufficient or pace has been too aggressive in the early stages.
Yes, but the finish will be slower than planned. Taking on carbohydrates immediately, slowing to a manageable pace, and walking where necessary will get you to the line. The body cannot fully restore glycogen mid-race, but reducing intensity allows fat oxidation to partially compensate.
Carb loading maximises glycogen stores before the race and significantly reduces the risk. It is one of the most evidence-backed race preparation strategies available. It works best combined with a disciplined pacing strategy and consistent gel intake during the race.
The brain is almost entirely glucose-dependent. When blood glucose drops sharply, cognitive function, decision-making, and motivation all deteriorate. This is why hitting the wall feels mental as well as physical: runners describe confusion, despair, and an inability to think clearly alongside the physical breakdown.
Yes. Improving aerobic capacity over many months increases your ability to burn fat at marathon pace, which spares glycogen. Practising fuelling on long training runs trains the gut to absorb carbohydrates efficiently during exercise. These two adaptations together substantially reduce the risk.
Summary
Hitting the wall is not random. It is the predictable outcome of running too fast, fuelling too little, or both. Every element of it is addressable through preparation: training your aerobic base, practising race-day fuelling, loading carbohydrates before the event, and executing a patient pacing strategy on race day. Runners who have hit the wall and understand why it happened rarely hit it again.
If you want to understand exactly what your marathon preparation should include, the services page explains how JM Coaching builds race-specific programmes. For runners who want a structured plan without ongoing coaching, our TrainingPeaks plans include built-in fuelling guidance for every long run.
Founder of JM Coaching, 2:08:45 marathon runner and Great Britain international. Coaching runners of all abilities since 2013. Read his full profile.
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