Depends on what you're going for. Are you planning to race at some point and want to get faster for that? Or would you just like to go faster so you can get to a particular destination faster? If not, who cares what your speed is if you're getting the fitness benefits you want and enjoying your rides?

My speed is slower than yours, though it's improving without my even trying, just because I'm riding more and improving my pedal stroke and aerobic capacity. I wouldn't mind being a little faster, but it's not really my goal. My goals are fitness and joy--if I'm not having a good time on the bike, something's wrong, regardless of speed. What I do measure are time, distance, and heart rate--those seem to be good indicators of fitness for me. But your goals might be different from mine.

Having said all that, 13.5 mph, esp. with the headwinds you mentioned, sounds pretty respectable to me.