Anyone who has trained for a marathon has heard the warning: watch out for mile 20. Runners describe it as their legs turning to lead, their pace collapsing, and a wave of exhaustion that feels sudden and almost total. It is such a well-known phenomenon that it has its own nickname — hitting the wall — and the science behind it explains why it happens with such precision.

Your Body Runs on Two Fuel Tanks

During exercise, your muscles primarily draw energy from two sources: glycogen (stored carbohydrate) and fat. Glycogen is the fast, efficient fuel your body prefers to burn, but the tank is small — the average trained runner stores only enough to fuel around 90 to 120 minutes of sustained effort, which lines up almost exactly with the 20-mile mark in a marathon.

Fat, on the other hand, provides a nearly unlimited energy supply, but it burns much more slowly and requires more oxygen to convert into usable energy. When glycogen runs low, your body has to shift toward relying more heavily on fat, and that transition is neither instant nor smooth.

What Hitting the Wall Actually Feels Like

When glycogen stores are depleted, several things happen at once. Blood sugar drops, which affects brain function as well as muscle performance — this is part of why the wall brings mental fog and a loss of motivation, not just physical fatigue. Muscles that relied on quick glycogen-based energy suddenly have far less available, so pace naturally collapses even if the runner is pushing just as hard mentally.

Many runners describe it less as gradually slowing down and more as suddenly hitting a physical ceiling — hence the name.

How Runners Push the Wall Back

• Carb-loading before the race increases glycogen storage capacity in the days leading up to the event, effectively giving the body a bigger tank to draw from.
• Fueling during the run with gels, chews, or sports drinks replenishes glycogen in real time, delaying depletion.
• Pacing strategy matters enormously — starting too fast burns through glycogen stores much quicker, which is why experienced runners deliberately hold back in early miles.
• Training the body to burn fat more efficiently through long, slower training runs can shift the ratio of fuel sources used during a race, stretching glycogen reserves further.

Frequently Asked Questions

Why does the wall usually hit around mile 20?

Trained runners typically store enough glycogen to fuel roughly 90 to 120 minutes of running, which for most marathon paces lines up closely with the 20-mile mark.

Can you prevent hitting the wall completely?

It can be delayed or minimized through carb-loading, in-race fueling, and pacing strategy, but eliminating it entirely is difficult since it depends on how efficiently an individual runner is trained to use fat as fuel.

Is hitting the wall dangerous?

It is generally not dangerous on its own, but the combination of low blood sugar, fatigue, and impaired judgment means runners should slow down, fuel, and hydrate rather than push through at the same intensity.

Hitting the wall is not a sign of weakness or poor willpower — it is a predictable, physiological fuel-supply problem with a real solution. Runners who understand the science behind it can train and fuel smarter, turning what feels like an unavoidable crash into something they can plan around.