You have some talent. What year of HS are you, when did you start training?
Try to listen to your body, and make sure that you do some days super easy or off - these are recovery days - and in between some days faster where you're not tying up with heavy legs but still moving fast and feel good.
How did you feel the day after the long run? Did you feel back to normal 2 days later? 3 days later? If you're still sore more than 2 days later it was probalby too long a run to be very productive for 800m-5K, and possibly too fast. It's pretty cool that you have the willpower and talent to run 20 miles though - this tells me that once you get training consistently you'll do well in long distances too.
For recovery days, try to run as effortlessly as you can. Think about it, you can run 1 mile in 5:03. That's moving pretty fast. So running a mile in 7 or 8 or 9 should be super, super easy. You should barely be breathing much and should feel almost like walking. That's the fitness you want to get to. So on your recovery days, start out running almost as slow as you possibly can, while still having a normal running motion/technique. See how your body feels, is it as easy as it should be? You'll naturally speed up over the course of the run, but keep trying to move without putting in effort. Eventually, your "effortless" pace will get down to the 6 minute range, but will take months/years.
If you "cheat" and run medium-fast all the time, your "effortless pace" won't improve much. Your 800m/mile/5k will get faster for a while since you're still relatively new to this training, but you'll lack the ability to go out and run 5-10 miles easy on days between workouts, and higher mileage will be very difficult since you won't be recovering fully.
In the mean time, your faster days and workouts will improve. But don't worry much about pace, you really don't need a watch until the season starts and I recommend leaving it at home as much as it's tempting to see your mile splits. When your legs are fresh and your stride is bouncy, run a bit faster, maybe do some strides and finish the run quick, but don't let your legs get very heavy every day.