Since I moved to SF a few weeks ago I have been astounded by the amount of off-road riding available within a few minutes ride of my front door. The Richmond district may be a little out of the way and have a foggy reputation, but the riding situation is great. My own avenue runs into the Presidio at one end and Golden Gate Park at the other, with both Land's end and Marin within a reasonable distance.
I have come across trails of all kinds in my explorations, from tiny connectors to full on sweeping single track, muddy double tracked service roads and sandy beach bluffs, urban obstacles even full on gnarly trails! Here's a taste: