We walked a lot while we were in San Francisco.
I believe we did around 10 or 12 miles per day.
Neither of us are joggers, so running was an unrealistic option for exploring the city in all it's quirks, rental cars are expensive (and parking even worse), and the bike rentals seemed like a great idea until it came to all those beautiful but steep hills.
So, we walked.
Sure, we also took the BART, the busses, and the occasional trolley car from time to time, but mostly we walked.
San Francisco is a city that, despite it's hills, is probably best explored by foot. The neighborhoods aren't nearly so far apart as they might initially appear; walking from Japantown to Chinatown took us maybe an hour and that's with frequent stops for photographs along the way and sipping some of the green tea I picked up at Nijiya. Fisherman's wharf was a quick trolley ride from there.
Even our sojourn from the Golden Gate Park to the bridge wasn't terrible, and we'd already walked to the park from our hotel near Haight.
Walking gives you the opportunity to see things you might miss out on when zipping past in a car, or to stop at your leisure to smell the flowers or admire the view without worrying about causing a traffic incident. On our walk from the park to the bridge, we passed a small produce store where we picked up sugar dates, peaches and fresh Rainier cherries for fresh snacking during our hike.