Ultra-endurance cycling like the race you mentioned is fascinating. I once met a few contestants in a European bike race and drove alongside one. He said the best in the field sleep around 4 hours per night and do 500+ km per day.
Then at the other end of the spectrum there are lots of people who go quite slow, but for months. I'm not sure which is more impressive; both show tremendous grit but in different ways.
The only thing that truly sucks is when you can't continue. Like getting two irreparable flats 20 miles away from town in rural New Mexico, or getting an injury that ends your journey.
Hard to explain to people that don't experience it like that, but even if it doesn't feel good all the time, it's basically always still somehow fun.