Escaped from a work event for 3 hours this Weds. Grabbed 29er full suspension bikes at Pac Bikes in downtown San Fran on 4th. $70 for the day, 29er hard tails were out of stock ($30/day). We hit the Embarcadero and up the Golden Gate Bridge to Marin Headlands (and back).
SF has great bike lanes and wide paths, but shared with runners and lots of tourists. GGB ride is stressful because you are shoulder height to a major highway, wandering tourists and distracted by the amazing views. Take your time and enjoy it. 15 mph limit on the bridge for good reason.
Marin has wide fire roads, great shape, some gravel so downhill was dicey on corners. Not much single track. We only had an hour in the wilderness. Plan for ½ or full day. Took us almost 45-60 min to get through city & over the bridge. The Headlands has amazing views, only 10 miles from the city and you feel like you are hours away. Some trails forced me to walk up, lose gravel and heavy bike with steep road makes it tough. Weather was fantastic but prepare for very cold winds, then hot sun. Do not need full suspension in the Marin Headlands at all, only had a few bumps/jumps but great flow until you see lose gravel.