Focus on a basic strength and conditioning program but keep in mind that the Marine Core will be looking at three activities with which they will test you and determine where you are:
3-mile run
Pull-ups
Sit-ups (they may have switched to crunches now, so do a search on this or talk to your recruiter)
I would focus on these three activities several times per week because the better you test out on these three activities, the more respect you will get from your instructors.I'm not sure what they are looking at for the sit-ups/crunches, but I think you will get a top score for an 18-minute 3-mile run and 20 dead-hang pull-ups.
For the run, start on a progressive program of increasing distance.Go to a local track and run one-mile repeats with 3 minute rest intervals between each mile.Run each mile at a fast pace and as the weeks go by, gradually reduce your rest times to the point that you are eventually eliminating the rest and running 3 miles continuous.
Also, you will be doing a lot of ruck marches, so work some of these into your routine.Go buy a top-loading navy seabag, fill it with about 50 pounds of sand (wrapped in several garbage bags to redcue the mess) and go for walks with it strapped on your back.You may want to alternate a day of running (where you focus on your 3-mile runs) with a day of ruck walking.I wouldn't go over 5 miles on these.You just want to get a feel for this.
Two additional activities I would encourage with the rucksack/sandbag.First, use it to exercise with.You can do traditional exercises like presses or curls as well as conditioning exercises like shouldering the bag repeatedly or doing lift and load drills onto a picnic table or the bed of a pick-up truck.When done for high reps, these types of drills build a lot of endurance and improve the strength of your core muscles quite a bit.Old time strongmen use to do sandbag lifting as a means to keep strong when they didn't have access to weights.Also, it will build a more applicable type of strength as opposed to traditional weight training because the stabilizer muslces come into play more when trying to lift a constantly shifting bag of sand.Give it a try and you will be surprised.
The other activity with the bag is to load mit up with all kinds of stuff; canteens, books, clothes, etc.Practice dumping it out and then recovering all the items and placing them back into your bag as quickly as possible.This is an activity your instructors love to do.Hint; when you are in boot and you recover all the items in your seabag before the instructor yells "Too slow!", don't just stand around.Help one of the guys next to you fill his seabag back up.Sounds stupid, but it will really impress the instructors because they are looking for teamwork.
For the pull-ups, do them atleast 3 times per week.Look into Pavel Tsatsouline's Grease the Groove technique.Look here:
Also, ladder training has helped me a great deal with pull-ups.Look here:http://www.cbass.com/Pavel'sLadders.htm
Do your pull-ups STRICT!If you aren't starting from a dead hang and pull until the bar touches below your collarbone.Any less, and it is good enough.I didn't really see any improvement in my lats or pull-up numbers until I really focused on the range of motion.I now pull until the bar touches at my nipples.
For your basic strength program, I would just focus on a handful of exercises.In the gym, I would follow a simple abbreviated strength routine.Something as basic as deadlifts and incline presses or squats and dips is all you really need.If you do too much work in the weight room, your other activities will suffer.Keep it short, simple and basic.No maxing out or forced reps.The old 5 sets of 5 reps system would be a good option.
In addition to all the above, get to like the push-up, because you will be doing lots of them.Especially when you make mistakes.So follow your weight work and/or your pull-ups with some push-ups and maybe some bodyweight squats.
Sorry about the long post.