I participated in sports when I was younger. I was in excellent shape and felt great. I got older and became an actor, the sports and exercise stuff disappearing for several years. When I turned 17 I decided my life needed to change and that I wanted to become a SEAL. I did the wrong thing for my body type (Endomorph) by doing a lot of weight lifting and eating a lot. I lost fat, but gained a ton of muscle. This was not what I needed. I needed endurance and lung capacity over long runs/swims.
DIET:
This January I cut caffeine completely out of my diet, and as of recently I have stopped eating junk food. WHAT YOU EAT MATTERS. I have started losing weight and looking trim just by diet alone. I am taking an extreme approach to dieting starting on Monday (I'll explain the Monday later). I will be eating almost nothing in order to lose weight faster. THIS IS UNHEALTHY AND I DO NOT RECOMMEND AN ACTIVE TEENAGER LIKE YOURSELF TO DO SO AT ALL. Eat good food. Make an egg and toast sandwich for breakfast. Eat some fruit and your daily meat for lunch followed by chocolate milk or a "professional recovery" drink. Dinner should be small and contain fruit and veggies. Mix and match as needed, just don't over eat and don't under eat.
EXERCISE:
As I said about my exercise picking up, I was doing the wrong thing by heavy weightlifting and no cardio activities. I put on weight (in muscle form) and my endurance sucked. I decided that was not going to work so I have switched over to exclusively cardio with some body weight activities. For cardio I am running 1 mile a day (currently at about an 8 minute mile and improving) and the body weight stuff is all pushups and situps (I am the KING of situps). Starting on Monday (which is July 1st) I am going to be dedicating several hours of my time to working out a TON and eating less. Something you can do to increase your stamina and endurance is to run sprints for short distances. For instance: run 100 yards, drop and push out 20 pushups and 50 situps, and walk back 100 yards and repeat without a break. Sprints will increase lung capacity without burning you out. Also for distance running, run further than you need to for whatever activity you want to participate in, and get a good time doing that. You will build enough endurance that the lesser distance you will HAVE to go will be easier on your body.
Any sort of weight exercise you do needs to be body weight related. You will gain muscle too quickly by using machines/ free weights. If you need lean muscle for running it's not recommended.
HEALTH:
I am flat footed, so I have increased risk of injury to my legs. STRETCH. Do yoga or something to loosen up before running. It makes a difference, and tight muscles while running suck. Side-stitch is that painful feeling you get in your kidneys when you either haven't drank enough water or aren't breathing properly. You need to take quality deep breaths to prevent this because once you get side-stitch, it won't go away and your run will dramatically suck worse. It's also good to have proper closing and DECENT RUNNING SHOES.
EXTRAS:
Figure out what body type you are (it's pretty straightforward) and look for workouts related to that body type:
Find a running route from your house that is the distance you need:
http://www.runningmap.com/Good luck.
