First thing you shouldn’t do: take two months off and think you can just charge up and down the Bay Area hills for five hours with no consequences. Come Tuesday you may equate climbing stairs with the fond thoughts normally reserved for getting a tooth pulled.
Say you’re crunched for time and can’t spare more than about 20 minutes. Here’s what I’d do: find a set of stairs somewhere and just climb up and down for whatever time you can spare. Clock yourself and work on improving your time, getting more laps in the same time expanse.
Fifteen minutes on stairs is less than a mile of walking, but it’s all resistance training that uses most of the same muscles, tendons and ligaments you’ll use on the trail. It won’t give the aerobic training you’d get in a healthy hourlong walk and it won’t provide any of the upper-body workout you should be getting if you care about that stuff.
But I suspect it will preserve enough leg muscle tissue to minimize your post-hike punishment.