stacey, you might need more gears before you feel yourself getting out of breath - If I mash my way up a hill in a high gear, my legs are pretty strong and don't really complain all that much... it's slow getting up there though and I stand up at some point.

if I spin up hills in a low gear - my legs are more than fine, I go fast, but then I'm out of breath and sometimes get a cramp in my side on extended hills - it's a much more cardiovascular workout.