As a three year spinning geek and sprint distance duathlete (age grouper) I am finding that mountain biking is a great interval workout and is brutal on the quads and hamstrings. I have only been twice but the soreness lasts 2-3 days. It amazes me how the suttle changes in workout routines can be such a benefit. For me its all about mixing it up an having fun with it. Will be interesting to see how hard the hills seem this spring with continued mtn biking.