Well done overpitched.
I knew you would get it in the end. A word of caution, treadmills at the gym tend to be quite short so turning 'downwind' can result in you falling off the end (a bit like turning downwind into a mountain - your frame of reference changes very suddenly!)
Try it instead on an airport walkway or similar, you will see that once again it is quite easy to turn round and run in the opposite direction. Just beware that there is a tendency that as you do turn round, the apparent change in speed of the advertising hoardings gives you the impression that you are speeding up. It is not unknown that people trying this, subconsciously slow down and end up stopping on the walkway (a bit like losing translational lift when turning downwind at low level!)
Cheers
TeeS
Edited to sort my thoughts out (doh!!!)