To improve: YOU. HAVE. TO. RUN.
There are no hacks, special workout programs or alternative means to make the changes necessary for improvement: it's running. Of course we do more than just run (speed, strength, mobility, skill) but the #1 thing we can do to improve is to run.
HOW IMPROVEMENT WORKS
When you start training and accumulating time spent running, you're signaling to your body (and your mind, but that's whole other topic) that you are a runner. The human body doesn't want running to be difficult, so it begins to make adaptations in order to help the process of running easier.
The foremost response we get from putting in training minutes is the creation of more mitochondria in your muscles. Mitochondria is where energy is created- and obviously we need to create as much energy as we can in order to run personal bests.
You can think of mitochondria as the number of cylinders in a car.
When I took off to North Dakota for my freshman year of college, I did so in a 3 cylinder 1990 Geo Metro. It was an $800 beauty. Great gas mileage, but obviously- due to the fact it was a 3-cylinder car- very limited in power. It took a full pedal to the metal, a downhill slope and a mighty tailwind to get that bad boy to 60 miles an hour. It didn't matter if I had the best tires, the most expensive gas, or even a light load...my speed was limited by the car only having 3 cylinders.
For those of you just starting training (or who have taken a really long break, since mitochondria need to be constantly replenished) you are like my 1990 Geo Metro. At this point in time, your potential is very limited. It doesn't matter how fast you get, how strong you grow, what type of spikes you buy, etc...you're limited by your engine. However...endurance ability (especially the increase of mitochondria, called mitochondrial biogenesis) is very trainable. No matter what level of fitness you're at right now, you can make BIG changes to your engine; and this is done by a totally simple process known as, getting out the door and running.
Keeping with the car analogy, every single run you do (and frequency is important, so at least 6 per week) you are sending the message to your body that it needs to build a bigger engine: and magically...it does. Less magically, you're able to run faster.
After putting in the work, you no longer have to drive around the 3 cylinder car, but instead build a 4 cylinder, 6 cylinder or whatever...you're only limited by your commitment. Soon, you've built a huge engine (even though you don't notice any daily changes) and you have the potential to create the energy needed train and race at speeds you would have never before thought possible.
This process of consistently putting on more stress to the aerobic system (endurance) is the easiest way for a high school student to get better year after year after year.
And who doesn't want that?