My commute of 15 miles each way is entirely on MUPs, other than a few blocks on the road. Pretty flat, as well. I typically average 18 mph and have hit a 20 mph average on a rare occasion, usually with a good tailwind. 18 is not a problem. 20 is almost impossible. Too many areas where you have to slow down. I did a group ride in Florida a couple years ago where we averaged 26 mph for the first hour. However, I was dropped on the return ride, and was told the average was 28.