You aren't the only one I am quite slow myself, averaging between 12-13 mph if there are hills - though I have seen my average get close to 15 without hills or much in the way of wind. Granted, my bike isn't the fastest bike out there, but I learned to ride only 10 months ago.

Since I am in the same place as you are I can't add anything constructive, just letting you know that you aren't alone with this.