You have to run three miles. I used to run ten to fifteen a day. You might seek out someone who can teach you proper form. If you run inefficiently, you will get tired faster. Take long jogs of a few miles and learn how to keep going while conserving energy. When you think your about to die then you need to keep going. It comes from inside, dig deep. When we used to run we would sometimes go at a good clip, then push as hard as you can for fifty yards. You need to build stamina and endurance. I don't know your age, so if your getting older be careful. And never eat anything you want. You will burn off the calories but your arteries can get clogged and your heart may suffer. It's all about conditioning.
You could also try going a little further each time. Running machines help to build your stamina. You can run on one in front of the tv or try listening to music, quite often you can go a lot further than before without realising it. Eventually you can build to running a few more miles each week.
Also, you can just try extra exercises skills that help your legs build muscle.you can jog every morning to help improve your strength in your legs.you can make it fun by having your iPod with you while your doing the exercises or jogging.if you don't have an iPod or Mp3 player or anything like that,just bring along a friend to jog with you so that you can chat while your jogging.and if your not jogging then just read a book while your doing your other leg exercises.oh,and remember to eat nutritious foods and only eat sweets and fats on occasions.and if you want to build up other muscle just do different exercises and sometimes leg exercises involve upper strength.so remember eat healthy,stay active,and have fun! =)
Well stretch everyday or as often as you can for as long as you can. This will really help. Then maybe try running only a half mile, but faster, then gradually start going longer and longer. This is what I did and it helped so much it is unbelievable!
Keep running, run farther each day until running a mile is easy without breathing too hard. An ipod or mp3 may help while you practice. Also, if you have a pool, practice holding your breath as long as possible.