how long does it take to walk from marina to mission district?
I’m going to San Francisco next week. how long will it take to walk from marina district to the mission district? and will it be a lot of hill climbing? I plan to do this in one day: go from marina, to mission, to haight-ashbury, and golden gate park. is this a stretch?

You could possibly do all that by walking in literally 24 hours, but in a "day" there is absolutely no way. Plus, you’d literally be hiking from sea level in Marina to somewhere around 400 feet in Haight-Ashbury.
I recommend using MUNI streetcars and buses to get around for the day. The routes practically take you anywhere in the city. And, it seems like all your destinations are touristy, so you’ll have no problem getting to where you need to go in a timely matter. (=
Good luck! =D
If you walk everywhere, yes, quite a stretch. Haight Ashbury and Golden Gate Park, you could walk to from the Marina, but you’d have to start with climbing some steep hills, and it would take you an hour or two, and that’s if you’re a fast walker and not carrying anything. The Mission is even farther.
You won’t be able to fit all that into one day without taking the bus. Muni (San Francisco’s bus system) goes everywhere. Fare for adults is two dollars, and when you board the bus, you get a transfer that you can use to ride any other Muni bus for the next three hours. (Be sure to take a transfer even if you don’t plan to transfer–they sometimes have transit cops checking for proof that you’ve paid your fare. A transfer is proof, and you can be fined if you can’t show any.)
Muni also sells day or week passes for tourists (locals typically buy the monthly pass instead). If you take many buses, it’s a slightly better deal than paying the fare every time.
If you like walking and walk fast, yes, you could. I just mapped out/measured the distance… walked 2/3 of the distance from the Marina to the Mission today in under half an hour (actually I live a few blocks north of the Mission and walked to the west/GG Park end of the Haight, so similar to what you’re going for). I’ve also walked from one end of the city to the other in just under 2 hours. I’m a freakishly fast walker though… But it definitely won’t take as long as the people above said unless you walk really slow. I’d say, for the average person, about an hour and a half from the Marina to the Mission. And yeah, there will be a lot of climbing heading south out of the Marina.
That said, it may not be enjoyable. It’s definitely a lot of walking. Also keep in mind there’s then walking within each neighborhood (and especially the park). If you like walking and that’s your goal, go for it. But if you just don’t have any other means of transportation, you might want to consider the miracle of public transportation! map: http://ersur.org/links/graphics/muni-map.gif all-encompassing site: http://www.sfmta.com/cms/home/sfmta.php