First things first there are a few definitions of a fitness model but basically pick up a fitness magazine or take a look at some fitness websites and you will have an idea. To motivate you, keep in mind that fitness modeling is not like other types of modeling out there. Fitness modeling is about keeping your body in tip top shape and that means if you have the body than you can be 40 years old and still doing fitness gigs.
If you are interested in fitness modeling than in my opinion the best thing you can do is to submit your photos to a magazine. Also if you have the desire to or the will to, you should try and enter a bodybuilding contest or a fitness show just so you have something to put on your resume if you are asked what kind of experience you have.
A lot of fitness models are discovered at competitions and than are hired for magazine work and promotional work and other fitness related stuff. You should always think positive and keep in mind that modeling is a competitive industry but in the fitness modeling and fitness industry in general there is room for a lot more people.