I know it's not particularly representative of long distance running, but there is a documentary about the Barkley Marathon that reminds me a lot of 1200k rides. Usually I feel pretty good on a 1200k, but there definitely are some low moments. Particularly the first 10 hours of the second day.
There is also a documentary about the Trans North Georgia mountain bike race that reminds me a lot of randonneuring. Particularly the people that should stop talking and either eat or sleep.