20 miles is great! Good for you! I thought of a couple more tips you might want to know:

You should pump up your tires before every ride, and it's a good idea to clean and lube your chain after every ride. Learn how to change a tire. Change out of your shorts right away after a ride to prevent UTIs or yeast infections. A good quality chamois creme will help prevent irritation from an uncomfortable saddle. Wear gloves to protect your hands if you fall. Carry a cell phone and ID (I put my name and home phone # on a sticker inside my helmet).

Enjoy being 19! 40 is better