When I went through Boot (08, MCRD) there was a great deal of emphasis on high rep body weight exercises and long distance cardio. You will run, walk, or march several miles a day. The longest distance that we marched with an existence load (roughly 100 lbs, +/- 15) was 10 miles . Obviously if you can excel at those, you will have an easier time during boot camp.
That said, I went through boot with a guy that was a former bodybuilder champion at the state level (I think Mr. Idaho). His forearms were so big that he couldn’t fully roll up his sleeves until a few weeks in and after losing about 20 lbs (he did say he got leaner as well). This guy was so strong that he effortlessly picked me up (at around 200 at the time) and another Marine slightly heavier during a MCMAP drill. The Drill Instructors “loved” him and he received preferential treatment despite being a poor runner and not particularly good at pull ups either.
Once I got to MCT (Marine Combat Training) the emphasis on long distance cardio was greatly reduced. One of my platoon’s sergeants didn’t give a flying fuck about pullups or 3 mile times. He said they had zero relationship to combat and instead put emphasis on 800m speed and strength.
You won’t be able to run 3 miles in 18:00 and achieve an exceptional level of strength, but I personally wouldn’t be concerned with 3 mile time. As long as you can max pull ups and crunches (absolutely no reason why you shouldn’t) and post a good CFT score you should be fine.
For training I would focus on a mixture of limit strength work, bodyweight work, and low impact, long distance walking. If you can squat 405 (or even 315), any load you’ll be marching will feel much lighter. If you can do a pullup at +100lbs bodyweight you should easily do 20 reps at bodyweight, especially if you bother to do some high rep sets after your limit strength work.
I would honestly just do a strength program with some walking or sprinting/conditioning 2-3 a week. Make sure you get plenty of rest and eat enough for recovery.
Edit: You will lose weight in Boot Camp unless you are a twig on double rations. I weighed in at very bloated and disgustingly stuffed 195 (normal weight would’ve been 185-190) and dropped as low as 170 halfway through. For most of Boot I tried to eat clean, no sugars or milk, and lots of protein. You will be constantly hungry in Boot Camp no matter what you eat.