Fatigue is not very realistic!?

[brxbrx]

You're assuming that they're giant humans, which isn't the case. They'd have much stronger bones and muscles, and perhaps organs we're unfamiliar with. Indeed, Jotun giants are descendants of ice, aren't they? Who knows how their bodies work.

Although your post reminds me of the World of Tiers, by Philip Jose Farmer. He described how a Lord crafted his world according to various Earth mythologies. Thus, he included centaurs, harpies, and the like.
Farmer went into detail describing the centaurs and their cardiovascular system, and how a simple human torso just wouldn't cut it to power a horse's body. Interesting stuff.