I was wondering the same thing.(Time vs. Miles) I think that although there are some technique/position differences between the bikes, pushing pedals is pushing pedals. I'm using my MTB to get to a more road bike friendly weight.(21 lbs in 3 weeks with a lot more to go) And since I am just now getting back into riding after 5-6 years off. I've been using training tips from road cycling books (plus it was all I could find at the local library)