I actually find mountain biking a lot more exhausting than road riding, maybe it's the steepness of the trails or the difficulty... And that's even though really I don't spend all that much time pedaling except up hills.

All I can say is - it gets better! I usually have a lot of fun trying to go over things till I fall a few times and say OWWies. And then I ride very sedately and pick my bike up and carry it over things.