I'm really glad I read this thread. I do all my cycling, commuting and fitness cycling, on a MTB and was starting to feel inadequate as my average speed seemed pathetic to me (around 12mph). But obviously (and I suppose I knew this subconsciously) it has something to do with the bike as well as my generally weak legs!

I'm going to keep plugging away on the MTB until I can afford (and justify) buying a road bike - I suspect this will be a few years down the road though!