Almost every section of the Colorado Trail has great mountain biking. The Mount Princeton section of the Colorado Trail traverses the flanks of the Collegiate fourteener and is a quick section that I sometimes do on the way to or from Denver. While I describe it as a loop with the flat roads in the Arkansas valley, this trail would probably work as an out and back as well, although the first section would have some hike-a-bike going up from Cottonwood Creek.