In a general way, it's like with people - you need to push a bit to build fitness, but push too hard, and you do more damage than good. One way to check is with recovery - rest a bit, and check pulse and respiration after a certain amount of time has passed. If it hasn't come down to a certain level, the person/animal is thought to be fatigued, and it's time to back off or quit entirely. The organized endurance rides have periodic vet checks; any horse that can't pass the vet check is done for the day.how exactly do you know when you're pushing them too hard