see i had a similar problem im 16 and in pretty great shape but i didnt run very often. One week both of my friends were out on vacation, so i went for a night jog and i couldnt get very far at all till i had to turn back. First i canged my breathing from regular breathing to inhaling 3 times then exhaling (in in in out). this help astronomically and i got about 2 miles farther than my pathetic distance. Since then i increased my distance by 2 blocks each night. After 3 weeks of doing this I can run 6 miles and im finally ready for the pt with my marine corps recruiter.