Maybe desk is a bit generous for this laptop platform, but if your treadmill is similarly set-up and you happen to have the raw materials laying around you get complete this in a little over an hour.
One important note, this desk is not the most stable in terms of being able to take a good hit, so be careful or you might find yourself hop-scotching your laptop.
Step 1: Desk Parts and Pieces
1 - Board wide enough for your laptop, long enough to fit the depth of the laptop and extend to about the center meaty part of the treadmill console.
2 - PVC pipe, about 4", with an inner diameter large enough to fit around the supporting cross brace of the treadmill.
3 - Rubber bumper feet of your choice, these are used to keep the laptop from sliding off the desk. Use as many as you feel you need and remember they will be rubbing against the laptop, so you'll want something non-marring.
4 - Wood for blocking the board away from the underside of the treadmill console, anything you've got that doesn't angle the desk so much the laptop wants to slide forward should do fine. The blocking just has to transfer the load of the desk closer to the middle meaty part of the treadmill console.
Of course whatever screws you have that won't go all the way through the table top of the desk, a compound or chop saw and a drill and/or drill press.