One of the most scenic and isolated areas of Upper Las Virgenes Canyon is traversed by a single track trail that links the Upper Las Virgenes Canyon fire road, northwest of Bell Canyon, to the northern extension of the Cheeseboro Ridge power line service road. The trail, shown on this Park Service map, follows an old road bed through improbable terrain.
I like to do the trail east to west. Once the power line service road is reached the most straightforward option is to turn left, and follow the road up and over the ridge and down to the 4-way junction east of Shepherds Flat. Another option is to turn right on the road and follow it a short distance to another single track that branches left from the road and leads to the Albertson Mtwy. This (dirt) road leads west to a signed intersection near China Flat. From here it is possible to work back to Shepherds Flat at the the top of Cheeseboro.
Many variations are possible, but starting at the Victory Trailhead of Upper Las Virgenes Open Space Preserve, and following the course indicated in this Google Earth image, the first route (up and over the ridge) is about 17.5 miles long. According to my GPS, an alternate route through China Flat is about 19.25 miles long. Both are interesting and challenging trail runs.