Rose Canyon Open Space Park is located in San Diego Just off the 52, between I-5 and I-805. The trail begins as fun singletrack including a couple of skinnies over some water crossings, eventually becoming doubletrack paralleling the railroad. There isn't much (1) in the way of side trails to be found along this route.
Begin at the trailhead at the west end of the parking area. Follow the singletrack trail west, bearing right at any significant intersection. As you approach I-5, turn right heading north next to the concrete aquaduct passing under the 52. Follow the trail noting the railroad to the west. Continue north as the trail eventually widens to doubletrack and turns east, still following the railroad. After turning East, note the side spur to the South (right) leading to an alternate trailhead. Continue East; when you reach Genessee, make a jog north(left) to the 1st stoplight. Cross the road and re-enter the trail on the north side of the fence bordering the school. Continue East along the trail (avoiding the railroad - these trains cook right along and there have reportedly been many fatalities; there is no need to cross the tracks with the suggested route.) When you reach the 805, turn around and proceed back as you came.
*Warning: Don't proceed East past the 805. MCAS Miramar begins here, and trespassing is a major no-no.