East of San Diego and Julian in the El Centro Resource Area, the Oriflame Canyon Loop (aka Oriflame Mountain Ride) is an awesome 16 mile loop with 2,900 feet of vertical. There are nice vistas of the Cuyamaca and Laguna Mountains, and also the Anza Borrego Desert.
To reach the mountain biking trailhead from Julian, head east on SR78 for 6.6 miles. Turn right on unsigned Chariot Canyon Fireroad. The trail starts from unsigned parking area on road side prior to the Banner Recreation Ranch Store.
Also, check out the nearby26 mile Four Canyon Ride.
Overall, Oriflamme Canyon is the 47th most popular mountain bike trail of all 132 mountain biking rides in California.