So obviously I can do walking at various speeds and inclines (usually 2 mph at 4 degrees) or standing. I have an ergo standing mat that I put on top of the treadmill if I'm going to be standing for a bit. Whenever I get the rest of this office cleaned up enough, I'll look for a small easy chair for occasional resting or iPad use. Off camera there's also a stand fan, which you may need if you walk for hours and your office window faces south in Florida. (My office has always been hot, and I find that moving actually makes it more bearable.)
This is something I want to experiment with as well. I tend to play music and shuffle, dance and pace around for movement while standing.
The thing that keeps me from going with a treadmill desk is that I switch from casual tasks (email, etc.) into highly focused work (coding, writing) seamlessly and for the focused tasks I prefer to sit most of the time, and even unplugging my laptop from speakers, power, monitor, etc. to take it to a chair or lower table just feels disruptive.
In terms of back health walking is hard to beat though. That's why I do my desk dances or even stand on unstable surfaces (a firm pillow or piece of foam in the simplest case) - to "diversify" my standing posture.
Coding while walking works fine for me. As far as the mechanics, it's not hard to walk and type at the same time. 2 mph isn't very fast. I can't draw with the mouse while walking, so the occasional graphics work has to be done standing.
Mentally, the consistent motion sometimes helps with flow, sometimes not. It's hard to tell, but switching between walking and standing seems to be enough for me to support the various required mental states. I've been doing this for about two months now, and while I have no hard data, I've done what I consider to be some of my best and most creative work ever in the last two weeks, so worst-case the walking isn't too great an obstacle to my coding.
And this is indeed a huge improvement for my back over both sitting and standing.
I often pace while thinking. An ongoing frustration for me has been having to sit down or otherwise stop to record what I'm thinking about. If you can't relate to that at all, then the treadmill desk might not be a good fit for you, or at least not for the same reasons it is for me.
