Whenever I've tried standing, my feet ache within about 10 minutes. Walking I can do for much, much longer stretches. Either way, it's a big change to go from sitting all day to either walking or standing, and a good idea to blend it in to your work day slowly over a couple of weeks rather than force yourself to go whole hog from the the first day.

Now that I think about it, when I switched from sitting to walking, I got a tall sit/stand stool so I could switch a lot. It was a really cheap stool, and it put my butt and legs to sleep in 10-20 minutes. It was so uncomfortable that I would only use it when standing was starting to hurt, and I'd sit until sitting was starting to hurt, and then have to stand again. That stool didn't last long, but it might have helped me transition more than I thought.