I ran low 16's my senior year, mid 18's my junior year, low 20's my sophomore year, and mid 20's my freshman year. I didn't start training seriously at all until the winter after my junior CC season. Looking back, if I would have trained smarter I think I could have been mid-high 15's. My senior year was the first year in a long time our team didn't completely suck- we had 3 guys under 16:10 and 2 16:20-30. The previous years, it was an accomplishment to make it out of the district meet. 3 years later they won the state meet (Ohio, division 1). I talked to some of the younger runners and my coach a little bit, and they had done some things differently than when I ran. I know there was a kid who didn't make varsity his freshman year, and ended up running mid 15's his sophomore or junior year (I forget). So, you might improve quickly, you might not. It might take some people a while to figure out what works well for them in training, others might get lucky and respond well to what their coach prescribes.