I got a cheap but very small flat treadmill and put it under my standing desk. It took a bit of fiddling to get the ergonomics right but it worked. I could walk at maybe 3km/h while typing, up to 6km/h while reading.

I used it for about 6 months but it was never not distracting and I eventually gave up. I think it's much more productive to just take breaks and go for a walk/gym/swim and give your mind a break from work. If you want, put a treadmill or stationary bike in your office - or just a yoga mat - and take exercise breaks while listening to something.

On the other hand, it was an interesting experiment and I'm glad I did it. The treadmill only cost $100 so if you already have a standing desk it's not too expensive to try, and maybe it'll click for you. I also think that if you spend a lot of time in video meetings it could work well for that. I was working on a solo project at the time and it's not good for deep focus.

I replaced the four screws in the bottom of my laptop(air m4) with hex bolts retaining a FLIR neckstrap and homemade torso strap. It makes a great walking laptop but I would add I'd much rather some kind of AR setup for this as the T-rex-arms are really only good enough to type out a prompt before the cramping sets in

I cannot focus while walking, but I respect people who can.