I'm at the back of the pack, if I can even see the pack, regardless of if it is road or mountain. But, here is my two cents. I find that when mountain biking I get more of a full body workout. When I'm on the road, I rely more on my core and lower half. I suppose, if your upper body is taking a beating on the mountain bike, it may be sapping your energy quicker. It could be a matter of some more upper body training or even just greater amounts of nutrition when mountain biking to compensate for the fact that your whole body is involved and may utilize calories quicker.