I have a mountain bike that I ride to keep in shape between hikes. We have a nice little trail here in my home town. It's on the side of a hill, so it has plenty of up and down (for Florida) to get leg and cardio workouts. It does a great job keeping me in shape, until I broke my collar bone last January - which kept me from doing much of ANY exercise for a few months. Then, when I was recovered enough to start riding again (around April), my next crash strained something in my lower back ...
Mountain biking is a lot of fun, but I never had so many injuries riding my road bike - or hiking.