This is basically what I do. Well, it's a desktop installed semi-permanently on a desk that I raised with cinder blocks to be high enough to fit a treadmill underneath. I try to walk 5 miles each work day on it. I can only walk about 3mph before things get too wobbly to stay focused.

I've found that there is some sub-conscious mental block that makes it hard for me to start walking, but once I'm going I barely even notice that I'm walking - I can get lost in the work I'm doing just as deeply (possibly more deeply?) as when I'm sitting or standing.

I've tried to come up with some objective ways to measure my productivity walking vs standing vs sitting but haven't come up with anything concrete. If I had to guess I'd rank my focus level high to low in this order: 1) First 3-5 miles of walking 2) sitting 3) 5th - 10th mile walking 4) standing. Standing is much more strenuous than walking to me.

Great idea! If you don't have the time or money to set all that up, or you want something a bit more portable, you can also pick up a pedal exerciser/exercise pedaller (same thing, oddly). They're $25-$50 on Amazon, and you can stick them under your desk and pedal away while you work.