From Webster's Dictionary:

1 : PERMANENCE, DURATION 2 : the ability to withstand hardship or adversity; especially : the ability to sustain a prolonged stressful effort or activity <a marathon runner's endurance> 3 : the act or an instance of enduring or suffering <endurance of many hardships>

Equestrian Sport

In equestrian sport, "endurance" refers to long distance races (often 100 miles) ridden by one horse-rider pair. The course is set, and the athletes all start at the same time. Endurance races were part of military history and also have a place in the history of the American West. The Tevis Cup is a famous race from the east to the west side of the Sierra Nevada.

It is an international sport and is governed at the international level by the Federation Equestre Internationale (FEI).