If you have a standing desk but find it difficult to stand for hours, I highly recommend an under desk treadmill.
I picked up a Treadly treadmill a few years ago and have logged hundreds of miles of walking while programming. It's thin and can easily slide under my bed, or be propped up against a wall when not in use.
The rest of the day I sit.
If you walk while typing is it difficult?
What I DON'T walk while doing is - really cognitively challenging tasks OR learning something difficult/new, because that is markedly more difficult while walking.
If I really need to focus because a task is well outside of my comfort zone then I will almost always be sitting. Luckily, software development isn't always that kind of work (though it often is!)