I started with a mountain bike a year ago and it was really hard - i could barely ride 15 kms and i was shagged. Im a smoker so that doesnt help.
I upgraded to slicks and i didnt look back. I found that slightly over inflating the tyres made it easier. i could instantly do more kms and slightly faster
I recently moved to a full carbon fibre road bike(Giant Composite 2) - it flies.
Im still averaging 50kms or so per ride but im doing it easier and faster than before.
If you want to do kms and go fast then get a Road bike
If you actually want to get fit bung on some slicks and get rid of the shock forks and the heavier bike is the way to go. IMHO
Should also mention that riding near a beach where the errrr "view" is good really helps
