I'm a freshman in high school, and I just finished my outdoor season. I can confidently say I'm in ~18:30 shape for the 5K. My mileage right now is mid 30s and I plan on increasing it significantly over the summer to low-mid 50s. My PR from freshman year was 21:51 and I've since improved to a 19:11 on a slightly long road course right before outdoor (beginning of March). I definitely want to be well under 18, possibly even under 17 come the cross country season. What should I be doing over the summer in order to achieve this?