Walking desk seems like a terrible idea (how could you type, or read, when in motion plus distracted by trying to keep your balance).

Under desk cycles/bike, elliptical and other such devices are readily available (just search for those terms). You use them when sitting (presumably reading...not sure I could concentrate on writing code while doing that). I have a couple of friends who swear by them and indeed they are both svelte. One I know used to be over 300 lbs and this was one of her tools.