What does it when people say long distance runners have "hit a wall"?
user
"Hitting the wall" is a common expression in the sport of long-distance running, where it refers to an abrupt and strong physical decline in performance in the latter stages of a race.  This decline is often accompanied by the feeling of being completely exhausted, mentally and physically.  Hitting the wall can also refer to the abrupt end of a successful prolonged effort in other types of physical activity.
assistant
What does it when people say long distance runners have "hit a wall"?
user
This phrase is used to describe the point in a long-distance race or workout where you're very fatigued and struggling physically, and have reached the limit of your physical ability. This fatigue is often described as a metaphorical wall, which you can hit and no longer have the physical capability to continue. 

When to look for signs of hitting the wall and strategies to avoid it: 

Hitting the wall refers to a state of extreme fatigue and exhaustion, so it is best to take steps to avoid it.  Look for signs
assistant
What does it when people say long distance runners have "hit a wall"?