I have been a mtn biker for ever, I would say it is my favorite thing to do (well with clothes on ). I have some tasty single track right down the street from me and a really cool state park a 15 min drive away so there are plenty of opportunities to get out and ride. My question is how much is mtn biking a good substitute for road riding when training for triathlon? When mtn biking I wear a HRM and keep my BPM up, I may stop 2-3 times in a 7 mile ride and try to push the pace as much as possible, but these are inherently different disciplines. Do you guys think I am missing something by spending some of my training time mtn biking rather than spending time on the road?

I've got a buddy who almost never trains on his road bike unless he just feels like it. He MTB's all the time, and averages 21-22mph in Triathlons without any aero gear.