I'm out of shape and VERY slow right now! :P Besides, climbing rides are great for riders of varying abilities. The faster climber can summit and turn around and descend for the slower rider.
There is some good mountain biking out here. Closest to my house is El Moro and Laguna Coast. Then there's Aliso and Woods Canyon, Whiting Ranch, all of the trails in the Santa Ana Mountains, etc. Today we did San Clemente Single Track (aka "The Weedpatch") and there was some stuff that was way beyond my beginning mtb skills!