The Santa Ana River Trail is the longest singletrack in the Big Bear mountain area. It's a destination trail for lots of riders in the SoCal region.
For this section, park at the South Fork Fishing Area, and find the trailhead that parallels the highway going west. You'll climb a little, then it's mostly down from there until you hit Glass Road (paved). Continue your adventure by crossing the road and starting back on the singletrack downhill. Or just come on back for a great 12 mile ride.
The trail is always in great shape and offers great riding views of the canyon and mountains. The average grade is very manageable, yet steep enough to push the climb and go fast for the descent. One of the highlight features of this trail are the banked hairpin corners - fun!