I love to exercise, especially mountain biking. Riding over rocky terrain, steep grades, and dirt is a huge challenge!
I haven't mountain biked in years. What kind if bike would you recommend?
What kind? Well, I guess it doesn't really matter. Something sturdy that make it over unpaved surfaces, like fire roads with debris is what you want.
Do you like dessert or wooded trails more? 
Wooded trails, my dude! Trail rides in the woods are easier to do long distance, and/or if I want to ride for multiple days.
What's the longest wooded trail you've done?
The longest I've done was 80 miles. It was competitive, too, so I had to make it in 2 days.
That's a lot more that I've ever done. The most I did was about 15 miles.
Pffft, 15 miles? You jokin' me? I'm so extreme, that in Alabama, where I live, it's illegal to drive at my riding speeds.