I think that your first day in the city, you should walk up Market Street from Castro station and then take Embarcadero until you get to Fort Point.

It’s about a two and a half hour walk but you will probably take longer since it will take you past quite a bit of the city, from the giant gay flag to the splendor and squalor of City Hall to one of the largest urban malls into the US, past the financial district, to the Ferry Building, along the water to the tourist trap that is Fisherman’s Warf and then along the beach to the Golden Gate bridge.