The backyard ultra or last one standing is a form of ultramarathon race where competitors must consecutively run the distance of 6.706 kilometres (4.167 mi) in less than one hour, which is a pace of 100 miles per 24 hours. The race is over when only one runner remains to complete a lap. This runner is marked as the winner and only finisher, with all other runners receiving a "DNF" (Did Not Finish). The runner who completes the second-most laps in a race is often referred to as the "assist", in the sense that their effort is ultimately what determines how long the race is allowed to continue--there is no predefined end length or time in a backyard ultra, as long as multiple runners can complete each loop within an hour.
The backyard ultra format has gained a reputation for its grueling and unpredictable nature, as well as the camaraderie among participants. Runners must endure sleep deprivation, exhaustion, and varying weather conditions, making it a true test of endurance and mental fortitude.