Joe and 18B got it right, it is about discipline and attitude. I'm not talking about stand up straight, suck your chest in, perfect salute discipline - well that is part of it - but I'm talking about an internal discipline. You have to want it. The Army literally becomes your life - Active duty or Guard/Reserve it doesn't matter. And you have to want it. The discipline to get and stay in shape, the motivation to constantly learn, the desire to find the good things in a crappy situation (and there will be many, many, many, and many more of those). But I also remember a book where one of the contributers, when he looked back on it, he didn't remember the lost pay, the tour extensions, and all the other problems he had. He remembered the good things. If you can do that, then that is all you need. You live but once, so just make sure that your decision is not one that you will regret.
Now I didn't make it, but not for lack of trying or a lack of motivation. I went into it with a certain perception of what I thought the Army and Soldiers were, but that was quickly changed. If you do go, go with an open mind and everything will follow. But Anthony is right, if you don't like it but you've already signed that dotted line, tough. There is no quitting in the Army, literally. If you fail they don't just let you go. For some, its hell, but others it gives them piece of mind and allows them to focus on doing harder.
Jeff