Getting in shape for PT.?

Im in DEP right now for the ARMY and we have mandatory meetings we go to every Thursday. Its not really a meeting, its actually PT. yesterday was my first day there and i almost DIED! I couldn't complete the entire running portion of PT, i got this mive head ache and my chest felt like it wanted to explode so i had to stop for about 30 minutes while the recruiters nursed on me until i could continue "good thing this isnt the real deal yet". Physically i appear to be fit 5'8 and weigh 150 and im 20years old. Thing is, i havent been physically active for about 2 years "since i graduated high school". Common sense tell's me that i should start drinking more water and eating more healthy, along with running, stretching and doing push ups and sits up on my own time between PT. I might have just answered my own question but does anybody have any other suggestions i could add to that?