Cosmo Park has one specific trail called Rhett's run (dedicated to Rhett Walters), but also many many off shoots to the main trail. Lots of climbing, downhills, sharp turns, it's a good 1/2 day workout to hit all parts.
How to get there:
From Columbia, find Cosmo Park. Trailhead is near the back by the softball fields.
There is a wooden walkway over a marshy bog for part of the ride, makes for some fun.