I built a treadmill desk a few years ago and I love it. It's tough to get myself out of my chair to use it, but once I get started I like it.

It's definitely not an all day solution though. I usually start feeling tired after about 45 minutes. I start sweating at about 1 hour. After 1.5 hours, I need a break. So I go back the my sit down desk.

The basic idea is to avoid staying in one place for a long time. Treadmill for an hour or so, then site for a while. Repeat.