Sundown Rundown is held in Kempton, Pennsylvania.
Organized by Fifth Street Cross, it is a unique trail-circuit race format.
Similar to a Backyard Ultra, runners must finish each lap (approximately 1.6–1.7 miles) within time limit hours to earn the right to attempt the next lap.
The key difference is that, unlike a typical Backyard Ultra, which repeats the same lap every hour,
this race progressively reduces the time limit time allowed for each lap.
The first lap's time limit is 20 minutes.
After that, time limit decreases by 30 seconds each lap, and this pace never resets.
In other words, the required pace gets progressively faster as the race continues, demanding speed as well as pure endurance.
Each lap also runs in the opposite direction from the previous one, so runners experience the narrow,
winding course from a fresh angle rather than simply repeating the same view.
The race does not set a fixed finishing distance in advance.
After completing each lap, runners may choose whether to attempt the next one or stop and lock in their final result (final lap) at that point.
Once a runner decides to stop, they cannot re-enter the race, creating an added strategic tension.
Ultimately, the winner is the runner whose final completed lap is the latest, whether they are eliminated for exceeding time limit or stop voluntarily.
The total length of course ranges from approximately 1.6 miles to more than 35.7 miles.