Hello, i am an unite states marine, and i get out of breath around a half mile or so on my runs. i can run 3+ miles but breath very heavily. why?

Deconditioned?: What is your level of conditioning? If you are relying on unit runs for this type of physical activity, you may wish to agument this with runs that are longer. Are you passing all of your prt's successfully? If you are doing higher mileage and been conditioned for a lengthy period - then ensuring that there isn't a pulmonary issue would be prudent.

Sounds like u r sick: You need evaluation by a competent physician, a pulmonologist or cardiologist, before you blame yourself for being out of shape. You may have anything from a cardiomyopathy to idiopathic pulmonary hypertension to any of dozens of others, mostly treatable -- or you might indeed just need conditioning. Please do get seen.