A backyard ultra is a deviously simple race. Runners just have to complete a 4.167-mile course within a one-hour cutoff. And then line up to do that again at the top of the next hour. And again the next. And again. There’s no limit on how long the race might be, (neither time nor distance). The race continues as long as there are multiple runners still on the course. The only way to finish the race is to complete the course when no other runner can.
There can be at most one finisher (and winner), while every other entrant is a DNF (did not finish).
The fastest runner won’t necessarily win this race. And mental strength is every bit as important as physical strength. How long can you keep moving? If you fight hard enough to earn a DNF at a backyard ultra then you may have truly found your limit. If you win, you might still be wondering.