What is the best way to build stamina?

Hi. I am 14 years old. I am a male. I weigh 123 lbs. I am going to join varsity track. Recently i went to virginia and i gained some weight. Last year in school i was in 8th grade and i was able to run a mile in 4 minutes and 37 seconds. I think the weight i gained might affect my timing in a bad way. I would like to know how to gain more staimina though because the most milage i can run consistently is 13 and i would like to increase that to 15. Please help me.

What is the best way to build stamina?
First if you have enough money pay for a personal trainer. And if possible get one that has an intrest in running or track and field. Second try running up hills and in low oxygen enviornments such as mountains. If that is not possible try swimming. Swimming forces your body to learn how to control its breathing. If you swim enough your lung capacity should expand leading you to more stamina. Don't forget to keep doing what you are doing because for a Freshmen in high school your time is excellent. What ever you do keep training.
Reply:Running stairs and adding weights as you get used to it.
Reply:you gain stamina by always moving... run for a while and play a good game of basektball or something and always move. just keep moving and make ur lungs work.
Reply:well if you just start adding the amount that you do to train then that should help
Reply:Eat foods high in vitamin B. This really gave me the edge when I used to fight in karate tournments. Keep up your training.