The only good thing to tell her is the 100% honest truth. Anything less is doing you both a disservice. There will be long separations - even if you aren't deployed, you will be away at schools, you will be training, or you will simply have long days at work. You won't be able to come and go as you please - you will have to follow the leave/liberty guidelines of your command. Sometimes, your duties will have to be #1, regardless of whether or not you put family first. You will move a lot. You might live in the best places on earth, or the worst places on earth. You will never be rich.
It comes down to Love. Love conquers all, right? If you love each other enough, you will collaborate (not compromise) and it will work.
Make no mistake about it, the military is a hard environment to be married. However, it can/does work, and the military has many resources to ensure that marriages are successful, such as the spouses' clubs. They are a great way for people in similar situations to get together and share experiences.
Another important, yet underused, tool is the command chaplain. Whatever your faith (if any at all), the chaplain is there to help you out/answer any questions. He will not try to "convert" you or anything, and whatever happens in his office, STAYS in his office. Just like a lawyer, he cannot divulge anything you say to him without your permission. It is absolutely separate from your command - they will never know unless you tell them. He is a military man (most likely married) who has gone through the same hardships as you, so he can speak from experience. He is always available, whether you are going through good times or bad. The chaplain's office often holds marriage seminars, marriage retreats, etc. to help with the biggest obstacle to any healthy relationship - COMMUNICATION. (Funny how one of the most important things in war is also one of the most important things in marriage...)
In any event, have frank, honest discussions with her. Do not hold anything back, and do not mislead each other because you want your relationship to work. Better to find out if it will work before you recite your vows.
Hope things work out, and stay in touch with us. And, as others have said, put her in touch with the spouses on this board. They all have different, yet similar, experiences to share that will help her decide if this life is for her. And if it is, the spouses' perspective will help her (and you) adjust to the rewarding life you are about to enter.