I enjoy walking in San Francisco, especially on weekends when the streets are extra busy with tourists and locals. I live in East Bay, so I hop on BART (our rapid transit service), right near my studio, and zip on up to the city in about 30 minutes or so. Couldn't be more convenient. I like to get off at the Powell and Market cable car turnaround to start my walk. Such interesting characters! Street performers are always there, and the ever present line of tourists waiting patiently to take a cable car ride up Powell Street and down to Fisherman's Wharf. (The going rate is $5). If you're looking for exercise, there's no better place to walk than the hills of San Francisco. And they're not for the faint of heart. It's just a whole lot of fun to walk and people watch at the same time, plus the health benefits are obvious. After a good long walk up and down the streets, your legs will be screaming for relief.
If you ever plan to visit our great city, by all means, email or call in advance. I'd be happy to take you on a "leisurely stroll." (And I also know some terrific restaurants where we can take a momentary rest.).
UPDATE: Here's a link to a story just published in our San Francisco Chronicle about a man who walked every street, alley, cul-de-sac (you name it), in the city. It took him 500 hours over the course of seven years!