Probably Marines.
Right now I'm doing one day of push-ups, one day of shoulder exercises (My main weak point) and one day of stairs (one hour).
That is a good start.
Now work in a weight routine that hits your core. Nothing sucks more than a heavy pack strapped to a weak core.
Run. Run. Run.
Without a strong heart and lungs, strong muscles won't be able worth a spit.
If the Marines are something you are looking at, pullups and use a lat cable after you can't do any more.