One sweet ride for the self-propelled is the 30-odd km out-and-back "Rafters to Coliseum" route at Wilderness Tours in Beachburg, Ontario.
It isn't officially that long, but the route is poorly marked, the online map sucks, and I ended up playing around on some extra loops anyway because they were such a rip. Really, these days there's no excuse for poor trail marking what with GPS, Google Earth and all--especially since a paid trail pass is required ($5-10 I think). But enough griping. It's really worth a spin. When I rode it last fall I had the place to myself and the staff generously refused to accept my payment for a trail pass. Once I hit the dirt I didn't see another soul for hours.
I think one of the top Zen moments was flying through this pine stand on a smooth, gentle downhill. The light filtered softly through the treetops, somewhere a woodpecker hammered away, and there was just the zing! of tires on a bed of pine needles. I had to ride it twice just to be sure I wasn't dreaming.
There's a network of shorter loops near the Rafters lodge area that dip in and out of a broad ravine. Downhills are screaming and the uphills are equally screaming. Bring a lot of water--it gets hot, humid and dusty, and there's opportunity for a lot of steady exertion. There's also a merciful chance to rinse the sweat off in a dip at the rapids.
I rode Kenda Nevegal tires, which aren't bad considering all the slimy clay you'll encounter if you get a sprinkle. When it's dry, it's hard and fast. I also recommend bringing a slower rider for company. I saw vivid proof of bears in the area.