Congratulations! Even though not all future rides will be that fast, it puts you comfortably in the mix for general group rides.

According to some training books I'm reading (my best is just over 16, so I'm not an authority!), doing full-power short intervals once or twice a week is the best way to increase your speed. And high cadence is supposed to help (I'm a novice, I start to bounce above 104, but I averaged 90 RPM today - it seemed to help).

Good luck hitting 16 in the next month or two!