I'm not understanding 20 min when you ran 6 x 800m with 1 min jog, all around 2:55.
4 miles at 7 min/mile is a great pace, especially for only a soph.
I am not a fan of workouts like 6 x 800m, especially in HS, but not even in college..
I don't use terms like tempo runs, or LT, or VO2max or any of the current trends.
Right now you should be able to break 19 min for 5k.
It doesn't matter whether you do, perhaps you are really more of a 800m/1600m runner.
I don't understand the fascination with CC in HS.
We ran to win, not because we loved running 3 miles fast.
"CC is for show, track's for dough."
I would stick to your current training pace, stay away from long runs (once a week, 1 hour at a pace that you could talk with your teammates while running, but not dawdling).
You can slowly improve to 6:30 min/mile for 3-4 mile runs.
Once a week interval sessions:
1. 600-400-200 with 200m jog, all at expected 1600m pace. 2 sets, 2 lap jog between sets.
2. 500-300-150 with jog equal to interval (500m jog after 500m interval, etc.). 2 sets, 2 lap jog between sets. At expected 800m pace.
Run (1) one week, (2) the next week. Repeat once and that is a month of decent intervals to combine with your solid distance running.