There is such a thing as a walking desk. Linus Torvalds uses one. Never used it myself, though. Couldn't imagine doing any serious programming like that.
If you can use a standing desk to program, you can probably get used to a walking desk. In the worst case you'd just need to stop walking when you start getting deep into some parts of your work. If you can't use a standing desk to program, then a walking desk is almost certainly out of the question.
Personally, I just accept that for 5-10 minutes of every hour I will not be at my desk and will instead be near my desk doing calisthenics.
Personally, I just accept that for 5-10 minutes of every hour I will not be at my desk and will instead be near my desk doing calisthenics.