You're far from the only slow rider. I know I'm a slow rider. I never rush on my commute, just kind of cruise.

But, that being said, I do it on a road bike (well, sort of, fixed gear road geometry). The road bike gets me further for less effort, and while this isn't necessarily a perk for weight loss directly, I find it makes me more likely to get on my bike instead of taking a cab/train. Also, the smoothness and niceness of it gets me biking more. When I rode a "mountain commuter" (cannondale badboy) years ago, it just wasn't very nice to ride so I didn't ride often.