I've got a sub-6mi (~12mi total RT) commuting route I frequently take, but at hours where there aren't a lot of cars on the road. I generally do about 11-12mph or thereabouts.

Once did a ~65mi out-and-back route along a river, on 95% MUPs and paved trails. Averaged 25mph for the trip ... and couldn't walk very well for the following couple of days, with the legs having stiffened up. This, during a time when I was typically doing 3-4 45mi trips a week at ~15mph average speed. That 25mph run was a one-time deal, and it was plenty.

Around my neck of the woods, I do occasionally see folks pairing up along a flatter stretch of road where they're doing ~25mph or so, but up through town that speed gets cut in half on most streets. And these are "racers" I see frequently doing the local club runs and "fun" races. Can't imagine the average good "commuting" cyclist does 20mph+ very often.