Maybe it's an unfair world, but models need to be thin. Unless you're a plus size model, no one is going to pay for an overweight person to get in front of a camera. Sorry, but you chose this life... them's the breaks.
That means taking care of your body, staying fit, staying lean, and looking good. It probably also means at least a moderate amount of time spent in a gym, fitness center, or working out in some way. You have probably figured out that you need a sustained cardio routine to keep your body fat low, but have you considered which exercise machine is the best for your needs as a model?
Let's consider elliptical trainers vs treadmills. These are the two most common machines in gyms today, and you've probably tried both.
As a model, I'd choose an elliptical machine. Why? Your perceived effort is lower, meaning you can exercise at a higher rate of intensity longer. Running on a treadmill creates a lot of stress on the body, which raises your heart rate, causing your perceived effort to be higher than it is. Plus, running all the time is hard on the joints, and if you're not training to be a runner... you don't need to take that pain.
So get to the gym, hop on an elliptical trainer, and spend 30 to 45 minutes working out at a moderate intensity. That'll keep you nice and slim, so you can keep looking good and getting work.