Here's me reporting back in after more than four months. Gosh, how time flies!!
Well, I found a cheap mechanical treadmill on eBay I could pick up on my way home from a longish car trip, but it took a while for me to really get going.
At first, in imitation of Shamrock Mommy's example, I tried to UR while doing all manner of things, like surfing the web or marking essays but after a few minutes it always becomes too hard to keep reminding yourself to go on walking while I'm concentrating and getting into the "flow" of surfing or marking (in my case). Thus, I think a mechanical mill is not suitable for any mentally demanding activities. I assume you need a motorised treadmill to keep you going after you enter the "forgetting / not wanting to bother to keep walking anymore" phase. Or maybe SRM is just better at multitasking than I am. After all, the word is that men are unable to multi-task.
So you might think I went out and got a motorised treadmill, but I didn't, as I discovered that I find listening to audio books (i.e. performing audio didactics) while DRing (read: Domestic Rangering) very satisfying and have had a high habit compliance with this combination. This way, I consider it a break / reward that I enjoy rather than something taxing like I did with the walk-marking.
Recently, I've added my facial tanning machine to the mix. I rotate its position to tan 1. face and chest, 2. back, 3. front of legs and 4. rear of legs on consecutive days. This way, I can tan while exercising and it's easy on the skin as I only tan the same area every four days at most (if I DR every day). I use it for 20 minutes at a time (I usually go on DRing for a while after it switches itself off) so as not to damage my skin. I enjoy this as the resulting slight tan makes me look better, the UV light improves my mood and in the wintertime it produces some much-needed vitamin D.
Okay, now you can all picture me doing this and laugh your heads off...
But: I score a lot of green days this way.