When you "hit the wall" during exercise, it's not just mental fatigue. It's a physical phenomenon caused by depleted glycogen stores in your muscles and liver. Experts say it usually occurs around mile 18-20 of a marathon due to prolonged high-intensity exercise. Symptoms include intense exhaustion, muscle cramping, and increased heart rate.

Source: https://www.livescience.com/health/exercise/what-happens-when-you-hit-the-wall