@joel I did this for a while. my findings: 1) eye strain and neck pain were the biggest problems for me. 3) height and hand positioning were the next biggest problems. 2) slower speeds with an incline made it easier to avoid issues. 3) after a few hours it was a bit much, and I didn't like standing on the treadmill when I wasn't walking. So I ended up swapping between desks to either walk or sit.

I did all this with a full size treadmill that became a permanent part of the standing desk. Maybe with the smaller modern treadmills and modern motorized desks, it'd be less of a hassle to swap between walking, standing, and sitting.

Over time I used the walking setup less and less and eventually just tore it down to get the space back.