Real mountain bikers don't putter around on pavement, which is why (if you identify with this description) you should try the following route on mostly dirt roads through the western foothills of the Cuyamaca Mountains. Allow three hours (or maybe a whole lot longer if you're inexperienced, out of shape, or want to stop for breaks) for the 20-mile loop trip shown on the map. You'll gain and lose a total of 3200 feet of elevation. Pack along two or more filled water bottles, and pocket plenty of quick-energy snacks to stave off fatigue.
From Highway 78 west of Julian, drive south on Pine Hills Road, Eagle Peak Road, and finally Boulder Creek Road to reach a convenient starting point -- the Pine Hills ranger station parking lot at Boulder Creek and Engineers Roads (take a Thomas Brothers map for this tricky bit of navigation). Alternately, drive west on Engineers Road from Highway 79 at Cuyamaca Reservoir to reach the same point.
From the ranger station, the first segment of the trip (5 miles) takes you south down the graded Boulder Creek Road, through pines and chaparral, to a sharp hairpin curve, where Cedar Creek Road intersects and heads west.
Segment two (four miles) on Cedar Creek Road takes you down along an oak-shaded stream and through a gorge that cuts ever deeper and more sharply into the bedrock of the Cuyamaca foothills. This primitive and occasionally deeply rutted dirt road is a kick for both four-wheelers and mountain bikers. At the bottom of the grade you cross Cedar Creek, full of clear-flowing water. A steep little climb ensues, taking you up to Eagle Peak Road. If you make a short side trip down the old dirt road to the left (now the trail to Cedar Creek Falls), you'll catch a distant view to the north of Mildred Falls, arguably the county's highest at 100-plus feet. After this year's poor rainfall, those falls may be little more than a dark stain on a cliff.
Segment three (eight miles) of the ride consists of an inexorable uphill grind on Eagle Peak Road -- first on unimproved dirt, later on graded dirt. You're home free when you reach the pavement on Eagle Peak Road. Turn right at the next intersection of paved roads and enjoy the fourth and final segment (three miles), a mostly downhill glide back to your starting point at the ranger station.