I started to type up a long post but my browser ate it. Chucklehead summed it up well: mojo. If you keep stopping, you lose your mojo. It sucks having to get started again. Your legs get cold and its painful getting going again, and takes a few miles to get up to full speed. The faster and harder you go, the worse it feels to start again after a stop. Now I didn't say non-stop... when I did windmill in 4:49 (moving time), we DID stop at 2 of the 5 rest stops.