Well I just got back from a week in S.F. and rented a sweet bike in marin county. It ended up costing $31 after tax with a clif bar. The bike was a giant Trance X1 with a carbon bar and XTR crank/derailleurs, felt pretty light and was perfect for those trails. I brought all my own gear including pedals. The kid at the shop said a DH bike would be required for some of the singletrack that is now closed.
i took it easy and just enjoyed the views, used a trail map and printed the directions on http://www.marinfattire.com/
I chose the epic twin peaks loop. Had to stop a bunch on the way down to take pics and make sure I didn't miss my trails, it would not be cool to miss your trail and climb back up.
It was pretty much all fire roads but I was there for the climb so that was ok, there were a lot of switchbacks and it was really steep, a couple of spots I really had to push hard just to make it up the frickin "road" other spots were washed out and narrow- good luck driving a truck up there, no thanks. The final stretch to the top was very steep and rough with "death cookies" basically a lot of rocks, it was cool with me.
Had an absolute blast, the descent was awesome, smooth, fast, and if you missed a corner it wouldn't be pretty. A decreasing radius corner almost got the best of me, oops. Can't wait to do more rides like this, long climb to the top and a fast ride back down.