You can get just as fit on an older Specialized Hardrock with knobby tires as you can on a $2000 carbon fiber Ultegra equipped road bike. You just won't be going as fast. Your body only knows how long you ride and how hard you ride - duration and intensity. You may want to consider getting a heart rate monitor to gauge your effort levels. If I were to have only 1 bike it would be a mountain bike. They're great to get started on for sure.
My cousin is one of the fittest people I know. He doesn't own a road bike. He can sustain 18 MPH for 2 - 3 hours on a hard tail MTB with fat knobby tires and he isn't a cyclist. He can do a 5K run in just under 18 minutes also. Wish I could do that.