The difference in speed between roadbike and MTB on the road is 50%. There is no wasted effort which is a lovely feeling.
Scott if you want to look at some bikes PM me, I'm only down the road and I have a garage full of the damn things.
If it was me (and I've given this advice a few times) don't buy new - there are loads of good bikes out there especially if you are an average size (I'm really tall so I struggle to get S/H). If you must buy new then I would buy a hybrid as light as you can with rim brakes not disc (discs need loads of maintenance and expensive maintenance at that) and no suspension. You'll be able to go most places and you'll be pretty quick on the road.
Do you not remember being a kid? I used to ride everywhere on a bike that was once my (female) cousins - no sus, no discs and nothing stopped me

(edited to add when I said the roadbike was uncomfortable that's not the ride, it's just the fit. I think I'm too fat. Or need a new saddle (any advice on saddles which don't leave you numb would be great. Advice from gentlemen if you see what I mean.)