Heading into your sophomore year after a good track season is maybe the best opportunity you will ever get to make huge gains as a runner. But it's about consistency, not intensity. Basically, in order of importance:
1) Run 6 days a week. No exceptions barring injury or severe illness.
2) Increase your weekly mileage gradually. That might mean increasing to 35 by the end of the summer, or that might mean 50, depending on where you're at now.
3) Once a week, do a moderate tempo run (20 minutes total, either continuous or split into 2-3 segments) at 6:20 pace, or a bit slower. As the summer goes on, you'll probably be able to go faster at the same effort. Don't strain.
4) Once a week or so, do some long intervals (800-1600) with plenty of rest at your 5K pace, around 6:00/mile, but it's OK to start out slower. These should also not be intense efforts.
5) Once a week or so, do something about speed, like hill sprints, or flat sprints, or 200s. Take lots of rest in between. You want to be smooth and fast, not straining.
6) Once a week or so, run 50% longer than you usually do. So if your basic easy run is 5 miles, run 7-8 miles.
6) Do lots of easy running on trails, roads, parks, hills, grass, beaches, and anything else you can find.
7) Get your friends and teammates to run with you.