I spent the summer putting 800 miles on my hybrid on the local roads and paved trails. Today I tried my first mountain bike single track course. It was a three mile long 'intermediate' rated trail with plenty of elevation and obstacles. Man what a workout. Great 'core' excersize. I feel like I did 100 sit ups. I have a WHOLE new level of respect for the mountain bike riders. At the end of those 3 miles I felt like I had ridden 30 miles on pavement.
The intermediate trail was tough. It scares me to think how hard the adjacent 'expert' trails must be like.
If you want a challenge (for a 55 year old newbie) check out the Reagan / Huffman Park trails in Medina (Cleveland) OH
Can't wait to get out and try it again.