Long distance I believe is the most demanding part of track.
You need to cut off carbonated beverages, Milk 1%, Orange Juice, and Water.
Healthy dieting is depending on you, so you'll need to look that up.
Take weights, up to 20lbs, and run with them if you're allowed, and use dramatic arm swinging. This improves arm stroke technique giving you better momentum.
Though you do long distance, you need to do sprints as well. Sprint as fast as you can for 200m constantly.
Other days you want to do hill climbs, run up a hill and lower your body but not to the point where you like an idiot and take short steps up. When coming down, have long strides and slightly leaning back.
Try to run 5m at a time when you can as well, that will build up endurance the most.
Lift weights, and squat.
Lifting weights improve arms, and squats build up legs.
Eat about 24g a protein a day and every other day exercise your lesgs
Hopefully this will help, I used these techniques in track as well
