It's awesome exercise. In Cleveland, we've got an indoor MTB park, and Saturday was the opening day. I borrowed my friends 15" Giant STP SS, and it's great for that. The only problem is that the gear ratio you think might work is going to be too steep. a 26x16 was too tall for me on everything except for down"hills", I couldn't even climb the smallest "hills" with it.
Then again, the bike is a DJ bike, and it's more like a BMX bike than an XC. What kind of bike will you be using and where?