I just got a mountain bike a few days ago.  Wide tired bastard for actual mountain riding, not pavement.    Havent ridden a bike since i was a kid.  I thought hell, i hike all the time, my legs are strong, this will be a piece of cake.  Nope.  Man the uphill stuff is really kicking my ass.  Its like a whole different set of muscles and lungs than i normally use.  Gotta say ive pretty quickly developed a newfound sense of respect for bike people.