Here is a treadmill desk attachment that lets you get some work done while walking on your treadmill. The Walk with Me desk has a 32″ x 11″ x 1/2″ top and can accommodate any laptop and tablet. You can remove the board easily thanks to industrial grade velcro.
Walk with Me is pretty easy to use. You can not only install it with a simple screwdriver, but can detach the board when you want to run.