Many have heard about the phenomenon 'hitting the wall' in long distance running, and attribute it to a sudden slowing of pace, usually around the 20 mile mark.
Fatigue most definitely plays apart in 'hitting the wall', but it is also partly down to the body running out of fuel. Your body's natural energy stores take the form of glycogen (the storage form of glucose), and are stored in muscles and the liver. When the body demands more energy, it begins to convert glycogen to glucose, which can then be used for energy. Once these stores run out, then body will look for other sources such as fats and proteins, which do not provide the athlete with enough energy to maintain their level of exertion and performance.
Other factors contribute to this phenomenon, such as the weather conditions and pre-race nutrition. Even the elite athletes can 'hit the wall', it comes down to preparation, but recreational runners are more likely to have to face the effects. Interestingly, males are more likely to experience hitting the wall than females, and this may be down to physiological differences and/or a more aggressive outlook to marathon running. Physiologically, athletes that hit the wall reduce their VO² max from 70-85% to 40-60%, and this would be largely down to reduced exertion, triggered by discomfort and fatigue.
Psychological outlook is another factor which has shown to have implications to athlete's hitting the wall. Those that have hit the wall previously, or have negative associations with running beyond a certain mile mark. Positive mindset coaching, self-talk, and relaxation techniques can help athletes prepare emotionally for what lies ahead, rather than slipping into a catastrophising approach which has shown to be detrimental to performance.