I think the best set up is the standing desk + elliptical/treadmill set up. That combines all the best research of a standing desk and the benefits of very low movement to increase brain activity.
All day on a treadmill is going to be bad on your knees, hips and heels. If you running/jogging make sure you really pay attention to your form if you are going for that long (which is going to conflict with your concentration). If you are walking, you have to be even more cognizant of your form because walking typically involves constant heel-striking.
Elliptical might be a bit better, but now you are sitting in an uncomfortable seat and the repetitive motion will get you sore quickly.
