Not all of the Backbone Trail is open to mountain bikers. Of the sections that are open, this is probably the most interesting and the most fun.
From the TH, bike west on the Backbone Trail.
@ mile 2.52 turn left on doubletrack (dirt road)
@ mile 2.63 turn right onto singletrack and climb the hill
@ mile 3.17 turn right at the road junction (note: turn left for a mile to get a great view of the Pacific)
@ mile 3.26 turn left and enjoy a great singletrack descent
@ mile 4.83 cross Encinal Canyon Road
@ mile 6.04 cross Mulholland Road
@ mile 8.58 you reach Etz Maloy Motorway. You can turn around here or go left for another .4 miles for great scenic views on both sides of the ridge.
Retrace your route on the Backbone Trail.
When you reach Encinal Canyon Road, consider this alternative shortcut to avoid a long climb up the singletrack you descended earlier (see TOPO map): turn left on Encinal Canyon (asphalt) for 1 mile. Turn right up a gated dirt road (Trancas Canyon Road) for a quarter mile to get back on your earlier route.
(Note: you can bail out of the last half mile if you run out of time or energy by going through the overflow parking ... see TOPO map.)