Ignore the averages quoted by the various STW supermen no two riders or routes are the same the main thing is geting plenty of time on the bike, geting used to regularly churning out the miles.
to put it in context my commute is 20 miles each way flicking through my sportstracker logs I average out at about 15mph on the way in of a morning on fresh legs, but 17mph on the way home after a day at work. same route but in reverse, I put this down to the steeper hills being nice quick descents on the way back.
Average speeds mean nothing on their own.
keep up with your riding don't focus too much on the numbers for now... do the exact same route in a month or so to see if you have improved.
Having said that I did have hairy legs 
