Honestly - I still think you are calculating something very wrong here unless your entire route is uphill? I swear I am not trying to sound like a dink - I took my 6 year old on a bike ride that was about that far the other day and she averaged almost 9 mph vs. your 10 mph and she is riding a 20" wheeled, single speed Trek Mystic girls bike. There has to be another explaination here because something just isn't adding up?

Are you counting time spent stopped to wait for traffic signals, etc in the total time?

Try using www.mapmyrun.com and enter your route - this site is awesome.