I am a dressage rider located in what is referred to as the East Bay area. I am not sure what you know about this part of the world so I hope this info is useful to you.
There are 5-6 areas where several barns are clustered in the greater SF area so are you actually living in SF or another adjacent city? There are 10 million people and many cities in this area. Getting anywhere in 40-45 minutes depends on where you are going to live and work. The traffic is as bad as NY. I would suggest that you get a paper map (yes paper) of the SF Bay Area as this will give you a better idea than the online snapshots.
What is your boarding cost range? FYI, in California, everything is a la carte. Board usually only includes bedding, stall cleaning once a day and hay fed twice a day. Everything else is extra so you either do it yourself or have a trainer that will have all this stuff done on your behalf for a price. Turnouts are typically very limited because of lack of space and there is no grass unless the turnout is irrigated. Stalls often have paddocks attached so the horses can go in and out.
Most of the larger barns have multiple trainers in multiple disciplines. Some require that you have your horse in a training program. Hope that this gives you something to think about so you are not too surprised when you get here.
Feel free to PM.