Since its on a wednesday, I would recommend hopping on Caltrain (commuter rail) south to Belmont, and hookup with the Passion Trail Bikes shop ride. You can probably get a demo bike from them as well, and a guided tour of some singletrack, and an awesome post-ride party with great people. The park that they ride is pretty small, but has a nice network of trails.
Riding from the city will yield beautiful fire roads by crossing the bridge, but not much in terms of technical singletrack. You might get some singletrack if you can find a guide around the limited singletrack within the City itself. Several people from SF ride Skeggs on wednesday evenings also, and that might be worth exploring, but that might be more complicated.
If you ride road, and insist on riding from the city, then I would probably recommend a road bike instead.
Riding from the city will yield beautiful fire roads by crossing the bridge, but not much in terms of technical singletrack. You might get some singletrack if you can find a guide around the limited singletrack within the City itself. Several people from SF ride Skeggs on wednesday evenings also, and that might be worth exploring, but that might be more complicated.
If you ride road, and insist on riding from the city, then I would probably recommend a road bike instead.