I run cross-country for my school, and I made varisty and ran 5K. I improved a lot over the course of the season, with my PR being 24.11 minutes. (This was my 2nd year of running but my 1st year of high school.) I want to lower my time to a 21 or 22 minute 5K. Are long runs over the summer all I need to do to get better?
I'd do a steady diet of 400's and 800's once a week in your off-season and pre-season training. Speedwork is essential at those distances and your goal race pace being ran at those distances in repeats. It can be done with consistent running, smart training, and don't forget out recovery being just as important as the running is.
Try doing 400's up to 2/3's or up to a full race distance. Basically 12-400's and a 200 to finish it would be your full race distance.