:d Well it's now a couple of hours since completing my "treadmill marathon" and I'm finding it increasingly hard to get up and down the stairs!
I completed it in around 4 hours 15 minutes - I'm a bit vague because I stopped at around 18 miles to change my socks and when I got back on the stinking treadmill it had reverted back to zero! So it was around the 4 hours 15 mark.
I know this was probably mentally a lot harder than completing a "proper" marathon but my word I wasn't prepared for how hard it got between 18 and 26. The gym staff were great, supplying me with drinks and supporting me any way they could. It really helped, you can only "people watch" for a certain amount of time.
Waiting to hear what my prize is now - better not be a gym tee-shirt or such like. I will not be amused!