i know this isn't the answer you're looking for but...run more, run, run, run. you have to train specific. try changing the way you run, instead if running at the same pace on level ground, try sprints for a 100-200m every km. running up and down steap hills is also good. If your lungs are beat before your legs then your cardio is weeker than it should be. cardio is very important in basic because the instuctors will push you harder physicaly than you'd push your self and the muscles will not be able to work if they can't get oxagen. changing your cardio as much as possible will keep your heart and lungs guessing and force then to keep up (become stronger). your can bike and swim, these are good alternatives every once and a while but you should run of the time. train the way you fight and when you think you can't take another step keep going because thats what it all about.