good luck with your quest. I can't really tell how regularly you are riding, but if you want to be able to increase your mileage, you're probably going to have to ride at least 4 times a week for quite some time. If you want to ride a century this year, it's going to have to start now. 15 mile rides will certainly provide a training effect, but you are going to want to try the occasional longer ride once that becomes an easy ride.