If you want an idea of the scale of the city, remember that the peninsula is only about seven miles across. There are hills to deal with of course, but you could probably walk from Soma to the ocean in 3 hours. I usually find it takes me something like a half hour to get from Soma to the Mission, maybe an hour to get from Soma to the Upper Haight. Soma to ChinaTown/North Beach is definitely less than a half hour.

I like walking around in San Francisco a lot. You don't particularly need me to guide you around, just pick a direction that looks interesting and do it... the one thing you might want to beware are the "scumzoid" locations, indicated below...