Actually, it is a whole lot more than riding fast...and that was pretty dang snippy.
I have been training to ride/finish an endurance or Limited Distance ride so while I haven't completed a ride yet I have worked at becoming an endurance rider.
One must complete a ride of a certain distance in a certain amount of time on a horse that MUST pass vet checks along the way and when crossing the finish line. If the horse doesn't vet out it is pulled from the competition. And often an average speed can be anywhere from 6-10 mph so hardly a flat out race. The horses heart rate must hit a certain count in a certain amount of time, must be able to move out easily with no hint of lameness or stiffness.
Quite a bit of conditioning and training of both horse and rider goes into Limited Distance/Endurance riding. I have a heart monitor on my horse to monitor his conditioning. My horse right now could complete a 25 mile ride with no problem. Not so sure about me!! I am riding a QH so while we can go and have a good time, he is not likely to be competitive. An Arab or Arab cross is the endurance horse of choice for serious competitors.
Hoping to compete in my first 25 miler in March.