One of the races is a series of loops that keeps passing over the top of the mountain, so some of the more elite runners just keep doing loops until they get to 50 miles.

If they hit their head or break an ankle or just hit the wall and can't go on, we pull them off the mountain. Usually 1 or 2 per race need a ride on an ATV out of there, and every couple of years we haul someone out on a stretcher.

My boss has done 50-milers (not trail run 50 milers) and has discussed 100-milers with many folks who have done them. According to him/them, there's not really a conditioning difference between the two: hundred milers just take a little more careful work to take care of your feet, hydration, and energy intake.