Many marathon and ultra distance runners will tell you how significant 'hitting the wall' is, and how it causes a significant slowing of pace. This typically occurs around the 20-mile mark, where all energy stores in the muscles and liver are depleted, causing a sudden onset of debilitating fatigue.
Truly 'hitting the wall' is not a feeling of generalised fatigue and discomfort, which is part of long distance running, it is a feeling of complete exhaustion and disorientation. It is generally a result of poor race nutrition, aggressive pacing, and insufficient rest before running. There is also thought to be a cognitive component too.
Male runners appear to be more at risk of 'hitting the wall' than females, and this is thought to be down to males taking more pacing risks. Runners are also more likely to enter this sudden onset of fatigue in the years preceding a personal best, reinforcing the relationship between risk-taking behaviour and 'hitting the wall'.
If you want to have a better race-day, then consider the following:-
Practice and remain consistent with nutrition strategies in training cycles.
Consistency within training cycles, and prioritise long runs if you need to drop a session.
Practice mental resilience on a weekly basis for those hard moments in races. Mantras, visualisations and mindfulness can be useful.
Get comfortable with being uncomfortable, the last 6 miles of a marathon are the hardest, so you need be tolerant of when your body is telling you to stop.
Practice pacing long runs, and familiarise your body to what different paces feel like.
If you feel yourself fading at mile 20, try and eat some carbohydrates and drink an electrolyte drink. The sooner you get the fuel in, the longer your body will keep going.
Piacentini, M.F. (2021). How recreational marathon runners hit the wall: A large-scale data analysis of late-race pacing collapsed in the marathon. National Library of Medicine. 16(5): e0251513.