Ha! Did it, and it so totally is just a mental thing running for that long. I had to stop at the lights to cross roads a couple of times: the first one I was doing the bouncing up and down like a real runner thing, the second one after 15 minutes I was just hoping the green man didn't appear too soon so I could have a minute's breather. It's the only time I've ever known that light to change so quickly!
The finding nemo "just keep running" works very well - thanks to the lovely person who suggested that. But I can't get it out of my head now...