I work from home. I built a treadmill desk in my basement. Each work day I carry my laptop down there and walk 6 or more miles while doing my normal job. I can work effectively up to about 3 MPH, but I usually stay around 2.6 MPH.

I've been doing this off and on for about 3 years now. I wish I had done it much sooner. (I've been working from home a lot longer.)