Is it naturally going to happen as I ride more and more, getting stronger with each ride, and pushing myself a little bit more each time I ride, OR is it something you really have to work at?

I see the club rides in my area and would love to participate, but my average speed is not 17-19mph for the B group. Maybe on flats, but in Atlanta there is no such thing as flat.

My average last year (first season riding) was usually around 12mph. So far this season I'm sucking wind at 10-11mph.

Am I going to get faster naturally, or do I need to incorporate some sort of training?