Can I walk between terminals 1 and 2 at SFO or do I have to use air Train? Do I have to reclear TSA?
You can always walk between the terminals outside of the secure area without having to ride AirTrain.
There is a secure connector between Terminal 2 and part of Terminal 1 at SFO that allows you to do so without going through security again. Note that this connector only connects the D gates in Terminal 2 with the C gates in Terminal 1 (formerly known as gates 40-48); to the best of my knowledge, you'd need to go through security again to get to the B gates (formerly gates 20-39).
I made a video showing all of SFO so you can see what you're up against: https://youtu.be/xCHw8lN4L2k I talk about how to change terminals even when you don't have a ticket there for the same day.