User: How long is a running ultra?
Assistant: The term ultra applies to any foot race longer than a marathon. Any foot race longer than 26.2 miles (or 42 kilometers) is considered an ultra marathon, or an "ultra". Some ultras are measured in distance. Common distances are 50 kilometers, 100 kilometers, and 100 miles. Other ultras are measured in time. In a timed ultra format, participants run as far as they can in a pre-determined amount of time. An example of common timed ultras is the 12 hour format, the 24 hour format, and the 48 hour format. A third category of ultra running races is called "Last Person Standing". These races repeat a 4.167 mile "yard" (loop) each hour until only one person can complete a yard by themselves. That person becomes "the last person standing".