Don't be so pessimistic, zerolegspeed. Like one of the earlier posters said, people do continue to develop after the age of 17! I trained reasonably hard in high school, but never ran faster than about 25/56. But I kept developing through college, and ended up running 3:42 as a senior. I *still* couldn't break 25, but my strength had improved enough to run 50.x relay splits.
There's nothing wrong with building up your mileage, but definitely don't neglect shorter interval work. You need both speed and endurance to succeed as a runner, over any race distance. Just don't pigeonhole yourself as a "no speed" guy and give up on yourself.