I don't know if it is still said on the endurance trail, but when I was riding, we had a saying: his brain is trying to write a check that his body can't cash. That's a tweaked version of the familiar phrase, but what it means is that the horse THINKS he's superman and can maintain that pace all day, but in reality, he can't.
While he felt like Superman, I wanted him to finish the day just as healthy as he started it. While Speedy gets worked daily, we don't do hills anymore, but that's what we spent several hours doing. So even though he wanted to blast down the trail, I insisted that he walk. And to my complete happiness, when I turned him out in the arena at home for his post-trailer-ride roll, he leaped up bucking and galloping. So much for a tired horse!