This brevet starts and ends at the Golden Gate Bridge. If you're not familiar with brevets, they're like organized centuries/doubles, only both less and more organized.
Less organized because there's minimal support along the way and no route markings. More organized because you have to check into several "controls" along the way and get time-stamped signatures or receipts to prove you were there. Then, if you like (and pay $10 or something), a French bureaucrat will eventually send you a tiny medal for doing the ride.
Even if you don't bother with the medal part (I never do), brevets are a lot of fun. If the weather's good, there will likely be 80 - 100 people doing this ride. The route goes from the bridge to Petaluma, then Healdsburg, then out to the coast and down Highway One to Point Reyes Station, then back to the bridge. Pretty flat for 180 miles, only about 7000 - 8000 feet of elevation gain, with no major climbs (but a number of short climbs and a lot of rollers).
See SF Randonneurs for more details and the registration form if you're interested.