BTW, your recruiter is lying to you. They have quotas to fill and don't give a rat's ass about your military career. Find someone who did the MOS and ask them. Signal/S6 is consistently some of the best post service careers followed by medical.

Unless you're fully intending on doing 20, stay as far from infantry as you can! If you want a career consider infantry.

Which branch? MOS? like other vets are saying RUN. You'll get a lot more credit if you max a PT test, also shooting expert rocks it. Of course kicking ass in your job doesn't hurt

Running. That was a huge emphasis when I was in the Marines. I think all branches focus on it though. You need to be a master of your own body weight and have really good endurance. Start going on long hikes with a weighted vest or backpack too. Push ups, pull ups, etc. Like I said, master your bodyweight and build up stamina. Do long, intense workouts. Get accustomed to working out to the point of puking lol.

CrossFit

Now that I'm off work, and have spent more time thinking about it...

I'd say practice some sprints... but also probably be able to run about 3 or miles at a good clip before you get there. That will save you some grief.

The officer program I went through was designed to be a shocker... and get people to quit (DOR, or drop on request)

I'll just say that falling out of formation during runs sucks. There WILL be people back there waiting to properly 'motivate' you!!!
Running in my opinion is the hardest part of these programs to catch up... I mean if you can only do 30 to 40 push ups right now without resting... in 3 days you'll be capable of more. A lot more. But 3 days isn't a lot of time to miraculously go from 12 minutes to 10 minutes and 30 seconds in a mile and a half, if that is where you are starting.

60 push ups in 2 minutes and 60 situps in 2 minutes would be a good idea too. More would be better.

If you are enlisting, they aren't there to be your mom, but they aren't quite as bad about trying to make you quit... they want to make you grow into a soldier. That basically is the Drill Instructors job.

You will find with a D.I. screaming at you intensely, you'll be able to do stuff you probably doubted you could.
Don't worry about "How many Sit ups do I have to do?" Or how many push ups... just go and go hard all the time. Drill instructors will stomp you if they think you are coasting. And you know what? You deserve it! You didn't join to coast. Coasting can get your friends killed. If they coast, you could die. They want to see that you are willing to put out more than what you think 100% is. I found out I was a lot tougher than I thought I was. For sure.

I had a friend who later went into the program I went through, and he always wanted to know numbers. I always told him "It doesn't matter, as soon as they see you gassing out on push ups, they'll yell 'MOUNTAIN CLIMBERS! GET EM!', and just as your shoulders are about to collapse from those, following push ups... "LEG LIFTS! GET EM!"!!! Its a never ending cycle of abuse and after a few weeks, it won't matter. It really won't. You'll become a machine. Almost everyone in your class will become one. My friend went down and quit. The initial running destroyed him, then he lost motivation and dropped on request.

I went into Navy AOCS like the movie An Officer And A Gentleman...

My advice for an officer candidate school would be RUN!

RUN LIKE A GUY WITH A KNIFE IS CHASING YOU.

We rarely ran more than 1.5 miles or so... so it was FAST. We didnt do 5, 10, etc mile slogs. Everywhere it was a uncomfortable pace. Why? We had too many classes like engineering, aerodynamics, etc during the day. We didnt have an hour to run. We had 10 minutes, so a 1.5 mile sprint was it.

Everything else just sucked... but no one fails because they can't do enough push ups.

I think any routine you do that includes proper cardio, some floor exercises and some resistance training is fine. Most people who enlist don't do anything to prepare. Basic Training will put you through a routine. Don't stress over it. I enlisted at age 18 (in 1979) in the USAF. I had no real prior fitness regimen going in and I got through it fine.

Your recuiter should be a good resource for preparation... a young man I know recently enlisted and for several months before he left for basic the recuiter would have morning p.t. sessions. I image they helped but I don't know how common that sort of thing would be.
One thing I know he learned was the importance of hiking with good boots and good socks! Feet are very important!!!

Have you looked at the recommended workouts at miltary.com? They have several different ones that are progressive...there might be people at your gym that are in the same place and maybe you could spot each other and encourage each other...I see guys in the gym working together to get fit for the military...it is inspiring to me since I am an Army brat myself. Good for you...and good luck...work hard...boot camp is hell, so I hear! lol