dsh, since an mtb has higher gi than most SS/FG's, weighs 15+ lbs more and has more rolling resistance, an hour on each regardless of what the final distance is will still be more workload on the mtb. And that translates into getting on a SS/FG or road bike for a ride after training with a mtb. Try it for yourself for a 30 mile loop or even an hour on either. I know after riding the mtb hard for a week or two hard, the next ride I do on either the SS/FG or vintage road bike, feels incredibly easier, almost like a vacation.