Brent being in good physical shape will help but the military (at least Army) will show you how to do everything you need to know. If you wanna do pushups and situps, find somebody currently in the military that can show you the proper way to do them (yes, there are whole books written on the subject). No sense in having to unlearn stuff you been practicing. AFA knowing your basic drill commands, doing them on your own won’t help much, you need to be in a group of 4 or more to get the timing right (don’t practice while watching Stripes :D) and someone who knows how to call out the commands. Don’t worry about those. You’ll be marching with the best of them with a few weeks training. Don’t even bother practicing shooting a rifle. I’m an expert marksman and had never even fired one before I joined. They’ll even show you how to stand up, sit down, and fold your clothes!
Keep in mind that the recruiters job is to get you in. There is no guarantee that you’ll be able to do the job he says you can do. Be prepared for that.
But go for it. Really, what have you got to loose?
Try to keep your head. Both of my brothers are out now (army, both of them) and they are both alcoholics.
It’s mostly due to lack of spine and personal fortitiude, but meh.
I apologize for not offering any concrete advice to this thread, but I think a quest for understanding and seeking accomplishment (a nebulous concept) can be rather dangerous, emotionally speaking. Getting yourself hyped to perform a difficult task (for a freethinker, the military is a very, very rough place to be) can be a set-up for huge disappointment, so please consider that. If the military were really “all that” everyone would join it.
Secondly, any quest to find one’s self is dangerous, too, IMNSHO. People of the sort who will go looking are also of the sort to think and assimilate opinions and ideas. IOW, your self isn’t static, but subtly (and not-so-subtly) changing over time. The quest is never-ending, and if you look to it as a goal to reach you will fail. It isn’t the tower of Babel, it is a never-ending staircase! 
The quest for self is kind of a catch-22. Those that wouldn’t look would find it easily, those that would are so mutable that they never find anything. 
I don’t want to sound depressing, I just want to point out that hinging emotional health on such important decisions voluntarily could be a mistake.
In order to put this post in some context, I am 26 (big deal, right?) joined the Navy right out of high school with pure intentions of going into nuclear physics (scored higher on their Nuke entrance test than anyone they’d had at that particular station), failed to get their security clearance, was busted down from an E-3 to an E-1in boot camp, not allowed to get discharged, and went from the prospect of getting a greate rate (military for “job” heh) to being a deck ape (the scrubs who keep the ship’s exterior clean and moor it up, etc).
Talk about disillusionment. My experience, while not unique, is probably not all that likely, either. But there is a chance that fate will screw you, and when it screws you in the military you really, really feel it.
Second story about the military, though somewhat tangential, I was friends with a guy who ended up being gay. I did not find this out by him hitting on me, but another sailor, and when word got out this guy was completely ostracized, harassed, and was actually beaten up for it. My idealism didn’t allow social pressures to dissociate myself from him, and I became ostracized (though not threatened) in the process. I make no claim that this was a good or bad decision on my part, it was just a decision and it had consequences.
At 22 you are reaching the age where there isn’t much time left if you do want to join the military, so sitting it over for a year or so will pretty much make things more difficult.
If there is one thing I have learned in life from my friends, it is that accomplishment is a chimera. If you don’t want to do something, don’t let someone tell you that you should. We aren’t all going to do grand things, and I mean that with respect to society and with respect to one’s self. I am sure you are a competent human being, but that alone is not what it takes to accomplish things. If you’ve ever felt that there was this one thing that if you just did it, or knew it, or knew what this one thing was, you’d be set… well, that is the chimera of which I speak. I don’t think it is possible, metaphorically speaking, to make a map of a thoughtful yet enjoyable life.
I’m not sure what else to say. I am very happy doing what I do now. After leaving the military I went to school and took up electronics (what I wanted to do as a Nuke anyway) and my work challenges my thought somewhat but not so much that I can’t leave it behind when I go home to play video games, learn programming, etc. So in that sense I have made good decisions in hindsight, but none of them were startlingly obvious at the time I made any of them.
All I require of myself is that I live through reproductive age and find a girl to have a child with me. Everything else is optional as far as I’m concerned. That is all the natural world requires of me, and I don’t see why man should require more. Some that know me say I have no goals and I waste my potential. What they mean is that I don’t have their goals and waste what they think I should do so that I can contribute to their goals. Ridiculous.
Do not join the military under any circumstances unless you are comfortable with the idea of dying for your country, and for all the people in this country, including those you dislike. Bad drivers? Maybe you dislike welfare. Maybe you find the current administration to be stupid (I don’t know, but I’d bet that there is some group or class of people that irritate you). If you can accept dying so that they may live, then the military isn’t the worst option at all; in fact, it can be a great one.
Good luck. Life ain’t so bad when you don’t require it to live up to your standards, and don’t think it requires you to live up to its. 
I was 24 before I found my “calling” and was 28 when I finally finished college. At the old age of 41 I am quite happy with my choice.
If you really think you would like the army, go for it. But please don’t join just because you haven’t yet accomplished anything.
Hey, I didn’t go until I was 25. It’s worth it. Every minute, no matter how bad you think it is.
And go Air Force. 