You don't even need to go that. You can simply exit via the "Arrivals" exit (rather than the "Connecting" exit) and walk out of the airport. From Terminal A, that means turning left after customs. In Terminal G, turn right.

Keep in mind that if you're not a US/Canadian citizen then you will need to obtain an eTA for travel to Canada. The fact your not actually planning to go to Canada isn't relevant - the airline won't let you onto the first flight without it. Plus of course the usual hidden city rules apply (except the baggage one) - if you have a return ticket it'll be canceled when you do this, and in the case of IROPS you could be moved to a direct flight or some other routing not via SFO.