There really aren't any limits, it's all about your consistency in training and how much you're willing to do. Here's my progression from high school, I attribute the gains mostly to increased mileage each year and consistent strength training. The track workouts were only occasional 10 x 400 workouts and progression runs. My training was nowhere near as 'smart' as many people do today, it was just pretty consistent hard work. I kept my 'easy' runs fairly quick, not a racing effort, but it certainly wasn't a hobby jog. I've since turned into a 13:40's 5k guy/ 28:30's 10k guy and I still stand by the easy runs not being a complete jog. Just make sure you don't bury yourself into a hole and listen to your body. Also, be aware of what your weaknesses are. If you have some natural speed, try to improve your tempos/long run speed. If you are a natural 'grinder' and rely on your ability to lock onto a decent pace, then work on your top end speed a bit more at that age.
Freshman: 4:59 roughly 40 miles/week
Sophomore: 4:36 roughly 50 miles/week
Junior: 4:18 roughly 60 miles/week
Senior: 4:14 roughly 60-70 miles/week
The biggest thing is be sure to run some mileage in the off season, you don't want to have to restart building fitness at the beginning of each season. Try to carry the end of season fitness to the next year.