I do all my miles on a cheap and heavy mountain bike. I've always actually wondered the calorie burn difference from a road bike vs mountain bike when riding on the road.
I'm dead in the water currently for anything other than mild exercise. I've got an MRI scheduled to take a better look at my back, and until then I'm supposed to be protecting it. I'd rather exercise more than cut back on eating, but right now I don't have much choice.