I have lived in the Inner Sunset for eight months now, and I'm just discovering the small, yet fun, trail network just blocks away. While it probably consists of a few miles total, the Mt. Sutro trail network is all single-track, tight and twisty with a bit of climbing, some rocky sections, but overall really fun for being so close. I could probably spend a couple of hours doing a series of small loops to keep my satisfied.
I found the trails through Strava, but spent my Sunday afternoon recovering from a big ride on Saturday. They were hard to find at first, having tromped through the woods, and possibly poison oak, before connecting to the main trail(s). It's great because it is somewhat hidden and just far enough from being accessible so they feel like my own private trails, despite running into a few hikers and trail runners.