i mtn biked for 6-7 years before starting road biking recently (month and a half ago). the difference is night and day. like C heath said, it's like a parachute on your back with an MTB. you're upright, have knobby tires, and on a heavier bike..which all adds up.
double in range wouldn't be surprising. you just want to keep hammering the miles out on a road bike
something that's a little hard to do with a mtb depending on what you ride (the trails i do usually dead end somewhere and i need to return back).
melloboy