Digital Gee

Okay, help me sort this out.

I have a basic MTB, the Trek 3900, with slicks. Nothing fancy, but it's a pretty nice ride, if I do say so myself. I put about 50-70 miles a week on the bike and hope to increase next year.

I am interested in a Trek 7.2, which is a road bike with flat bars. It is lighter, shifts more easily, and is generally more responsive in every way I can imagine.

Now, I ride for two reasons -- to have fun, and to get exercise. I believe when I eventually upgrade to something like the 7.2, I will probably ride a little farther because it's a little easier, and a little faster, as well. But will I get more, less, or the same exercise benefit?

In other words, if I stay with my current bike, don't I get a bigger excercise bang for my buck because it's less efficient, in a given amount of time? For every hour I spend on the bike, am I not getting more physical benefit from the less effecient machine?

Or have I forgotten to factor in something?