The USA Equestrian Rule Book defines Endurance Riding:
An Endurance Ride is a competition to test the speed and the endurance ability of a horse.
To be successful, the competitor will have to have knowledge of pace and the proper use of his horse across country. In an Endurance Ride any member of the Genus Equus counts as a “horse”.
1.1. The competition consists of a number of phases that are sections of the competition, which may be separately timed. At the end of each phase, in principle at least every 25 miles, there will be a compulsory halt for veterinary inspection. The Veterinary Commission should approve the distance of each phase of the ride. In a 100 mile ride there must be at least four Vet Gates plus the final inspection. The phases may be spread over two or more days, with each day being a minimum of 50 miles.
1.2. The total maximum time permitted for completion, including veterinary inspections and holds, shall be determined by multiplying the sanctioned length (in miles) by 14.4 minutes per mile. Thus a 50-mile ride would have a maximum time of 12 hours, and other distances would have a corresponding time maximum.
1.3. Whatever the order and rules for starting, each competitor carries out the whole competition as if he were alone and competing against the clock.
1.4. It is a competition against the clock. The horse that finishes the course in the shortest time will, in general, be classified as the winner of the competition.