Socal - San Diego County - Warner Springs NickNames: #LostValleyRoad/IndianFlats
This is a nice loop, 1/3 on a thin strip of rarely used paved road into the wilderness, with the middle third on dirt road through very interesting valleys and sections of wild SD county you never knew existed. To complete the loop you unfortunately have to return on Chihuahua Valley Road and Highway 79 - however, keep you eyes open, the highway is flanked by single/doubletrack a lot of the way - this will help to keep you out of the traffic!
This trail is outlined in the San Diego Mountain Bike guide (it's called Indian Flats), and a great description by Jerry Schad can be found at this link:
Here's the gist of it: Park at the intersection of Hwy 79 and Lost Valley Road (about 1.5 m north of Warner Springs). Travel up the pavement until the Indian Flats campground is reached (~ mile 6.7), then turn right onto dirt road. Take the dirt road all the way to Chihuahua Valley Road (watch out for a left turn at mile 10.3 that looks just as good as the right turn - STAY RIGHT). Left on Chihuahua Valley until Hwy 79, take a left and cruise back to your car. Total of 25 miles, but due to the technical easiness of this ride and the fact that a fair amount of the ride is paved, feels more like 15 miles. The absolute beginner, however, might be distressed by the steepness of some of the early sections...
Don't let the pavement fool you, if you live in SD County you MUST do this ride!