This is a little hike I took last week. Went down Polk Street out to Aquatic Park. Then through Fisherman’s Wharf out along the Embarcadero. Then back through Chinatown and Union Square.
According to Google Maps, it’s about a six-mile walk that can be done in two hours.
Also, not sure why, but Google Maps won’t map the route I outlined from California and Kearny to Post, and from Post out to Leavenworth, down to Eddy. Google Maps instead routes you from Kearny to Market, then from Market to Eddy. You don’t want to take that route unless you want to pass through some of the nastiest parts of the Tenderloin.