How fit do you have to be to join the US military? Do you have run fast, know how to swim, and so on to join.
Every service has its own physical fitness test that its service members must pass. For example, in the Army, a male who is 28 (such as myself), has to do at minimum 39 push-ups, 45 sit-ups, and run 2 miles in under 17 minutes. It’s not terribly difficult given that physical fitness training as a unit is done anywhere from 3 to 5 times a week in the morning. A score is drawn from your performance in each category; a minimum of 180 is required and 300 is considered to be “maxing out” (i.e. peak physical fitness).
The key is that while there is a bare minimum, no self-respecting Soldier shoots for that, particularly an officer. If you consistently come in last, others around you are going to have a little less respect for you, regardless of what job or role you have. The Marines are the most hardcore, mainly because they require pull-ups in addition to everything else, with the Army being the second toughest. I’m not sure, but I think ankle-grabs might be a large component of the Air Force and Navy tests. 
Air Force and Navy: not very
Army: sort of
Marines: hardcore
There’s a difference between fitness BEFORE you join and after you join. As long as you don’t have any major outstanding health problems it’s not hard to get in.
You go through basic training and they’ll get you into shape. Obviously once you’re in you have to pass the physical tests to stay in, but the armed forces are looking for health problems that will KEEP you from getting in shape, not really how good shape your in before.
I knew a couple of woman who joined the Army and they between the time they joined and were supposed to be inducted they put on too much weight, so the Army told them go away and come back when you lose the weight, which they did and they got in.
I knew a marine and she could barely swim. She said they tossed her in and she coughed and sputtered and made it out of the water alive but that was good enough for them.
I’ve seen quite a few recruits go in the armed forces and come out much better physically for it. I’ve know a few that couldn’t hack basic training and were sent home too.
So talk to a recruiter and ask him/her, they’ll let you know quick enough. 
The learning curve is pretty steep. We had several guys that lost 40lbs. in the 13 weeks of Basic and Infantry School. A couple lost even more.
You don’t want to be out of shape when you join. I was and Basic Training was much, much harder than it needed to be because of it. I ended up with bad shin splints and spent most of the course in pain.
It’s supposed to challenge you but being in shape is one way you can make it a little easier on yourself, the mental part is hard enough.
Overweight people are relentlessly picked on by drill sergeants. They’ll call you names, remove food from your tray, make you do extra exercises…
You have to achieve a certain number of fitness markers.
You also have to fall between their minimum/maximum weights and body fat %, which pretty much ensures I would never be able to join. A male friend had to put on 17 lbs before he went to Marine boot camp.
I’ve never seen that in all my 7 years in the Army.
This was common in Marine Boot Camp, at least when I was there in the late 1960’s. Those who couldn’t keep up with the training schedule were pulled from their platoon and sent to what was referred to as the “fat farm” where they would stay until weight went down and/or strength improved.
BTW, the fitness and intelligence requirements are increasing as a result of the bad economy. Speaking with Navy/Marine (enlisted) recruiters, the number of applicants has risen significantly. Once accepted, there’s just over a one year wait to enter boot camp.
Its kind of a weird story but I work at a college and the Air Force Reserve recruiter wanted me to enlist in the AFR as an officer and go through the Chaplain program that they have. I could run all they wanted, and I can bench press 285, do 50 sit ups, but my body fat ratio was too high. I have what they call “beer muscle”.
This was about three months ago.
There was a group of recruits at the YMCA where I work out. The recruiter was leading them in various exercises to get them in shape for boot camp, over the summer. AFAIK, they all made it.
Regards,
Shodan