Doing actual training on horses is a significant amount of exercise. Trotting alone is a matter of raising oneself up and down out of the saddle in order to keep pace with the horse's gait - essentially two hours+ of pure thigh blasting tribulation. Hunter/jumper and cross country take an insane amount of effort, too. If your horse is cantering or galloping you're keeping yourself suspended in the saddle in order to minimize impact, which will affect the horse's gait and even cause injuries to both parties.