Pine Mountain loop is much longer than China Camp front and back side combined if starting from the closest access at the top of the Fairfax to Bolinas Ridge road, maybe 2 or 3 times the miles, with much more steep and medium steep climbing. Bring food, plenty of water, and clothing layers - it's been cold. The climb out from the bottom near Kent Lake seems endless. Go clockwise when you get to the loop otherwise the climb out from the lake is straight up.
I like Pine Mountain for being able to ride flat out fireroad for miles from the top of Pine Mountain (mostly) down the ridge to the lake with no worry of crowds like most other fireroads and trails have in Marin.
If you start in Fairfax it's about 50% or more longer in miles and climbing either by Fairfax-Bolinas Ridge Road or via Deep Park School and Meadow Club fireroads, and you can return downhill beside doubling back the way you came either via Repack (quicker) or Tamarancho (including a few miles of tight singletrack), or passing the Tamarancho’s upper access, via the Ridge Trail (near new wide multi-use trail and fireroad).
Sunshine Bikes in Fairfax or any Marin bike shop has the topo map by Joe Breeze marking all these trails. But they might be closed on MLK day.
Any way you do it it's a long ride, give 3+ to 6 hours depending on pace and where you start.
