You can walk about 10-15 minutes down the North Access Road to a parking facility called PARK SFO, they have a green shuttle that will take you to the terminal, tip the guy a couple of bucks and you'll have no problem.

When you leave the FedEx facility take a left and walk the length of the cargo building and once you round the corner you'll see a tall parking structure on the right side across the street, just watch the traffic.