Many cycling clubs around the SF Bay area organize events to raise money for themselves, for charity, or both. Events are typically non-competitive, so the aim is to enjoy the scenery and generally have a fun day out. Routes are usually open to road traffic, so riders have to follow standard traffic rules. The advantage of taking part in an event is that you make friends on the ride and there are rest stops providing food, drink, restrooms and mechanical support. The most common longest distances are the metric century (100km, which is about 62 miles), and the century (100 miles). Here are some favorites.
The Redwood Shores Cycling Group are a friendly cycling group who host rides for a wide range of abilities, from regular no-drop beginner rides to more challenging adventures for those with stronger legs. You can find more information on Meetup. They also organize the Civilized Century, an annual event with a variety of distances, all of which are almost completely flat (hence the "civilized" moniker).
This annual event starts in Fremont and offers distances up to a century. The century route goes east over the hills to Pleasanton, Livermore and Dublin and back to the start. It's an enjoyable day out, and the volunteers and other riders are very friendly. It can get windy on the Altamont Pass, hence the many wind farms dotted around there, so I advise setting off early.
A hilly set of routes around beautiful wine country. Several routes options are offered, the longest being a century.