My new commute will be just under the mark at 14.5, but it is on a relatively flat valley floor (along the Colorado River). I'll be moving this Summer to within a few miles, so will be augmenting my shorter commute with longer road rides.
I've done a 25-26 mile RT commute 5 days a week, year round, and found it to be a great distance. 21 miles each way is a big step up, and I think a goal of 3 days a week is reasonable.