I'm not by any means a guru on trails around The Boat, but when I was there last year I hit up Coulton Creek, Scott Run, etc. trails in the area a bit northeast of town, near the town of Clark. I was with the wife, so did more like 10 miles, but the trail system was pretty extensive and I think you could easily make 15-20 miles out of it. The riding was mostly characterized by nice fast singletrack with some moderate technical sections. I don't think you could go too wrong riding out there. Also, someone's put out a nice topo map of trails in the area that I picked up pretty cheap at the bike shop in town. Might try to get back out there again myself this fall. Enjoy.