I love mountain biking, What do you think about it?
I really enjoy it!  I especially like to ride on mountain trails, singletrack, fire roads, and other unpaved surfaces.
Riding on the trails is great, Mountain bikes were in fact pioneered by Americans to be recreational. 
Trail riding is fun!  I like to ride on  trails, bridle paths, and forest roads, but not on roads regularly used by motorised traffic.
Yeah riding mountain bikes on the pavement is not fun. I think its so cool how Norway hosts the largest mountain bike races, They are over 53 miles long!
Wow, that's long!  That kind of riding calls for endurance, core strength and balance, not to mention good bike handling skills!