Hitting the wall describes the sudden, severe fatigue runners feel in a race, most often in a marathon somewhere around the 30 to 35 kilometre mark, when the body's stored carbohydrate runs critically low. It shows up as heavy legs, foggy thinking, and a sharp drop in pace.
It's the point in a long race where your body's glycogen stores are largely depleted and energy output drops sharply. Runners describe it as suddenly feeling like their legs turned to lead.
That's typically when glycogen stores built up before the race run low, especially if fuelling during the race hasn't kept pace with effort. The body has to shift toward burning more fat for fuel, which can't sustain the same pace.
Carb loading in the days before the race, fuelling steadily during the race with gels or drinks, and avoiding an overly fast start all help. Consistent long-run training also teaches the body to use fuel more efficiently.
Ease off the pace, take in carbohydrate as soon as you can, and try a run-walk approach rather than stopping completely. Pushing through at the same pace usually makes it worse.