what you all are forgetting is that most marines are muscular, which doesnt lend itself to long distance running. I am currently in training for OCS, before I knew I was going to OCS I was busting out squats, deadlifts, and other powerlifts on the regular, getting very strong and improving my sprinting ability, but losing my long-distance ability.
Being able to do 20 pull ups and run 3 miles in 18 minutes requires being skinny or else being in incredibly good physical condition. most of the guys talking about 20 pull ups and 18 minute 3 miles being easy are probably very skinny and cant squat or deadlift 300 pounds, something many marines are able to do even though they cant bust out miles in the 5-6 minute range