I have been road biking/commuting for a year now, out of enjoyment of the sport, but also with the goal of loosing weight. I have recently been wanting to ride taller and more relaxed, So I've been thinking about getting a fitness bike (Specialized Sirrus) to use as a commuter and paved trail rider.Today I took out my Trek 820 mtb, which I have not ridden in about 2 years. I quickly remembered how much I liked riding it (minus my loss of speed). Now my question is... Can i get as much of a cardio workout on it as I would on a road or fitness bike, even though I will be going much slower? It felt great climbing a few hills today, even though it wouldn't shift into the smallest front chain ring. So after a tune up, and putting on urban tires, will it serve me as well as a Fitness Hybrid would for commuting? I still will be expecting to go slower but that can be good for me, since it will force me to leave earlier and will more likely be on time. Thanks for any input.