Power
You're a fitness instructor; you're full of power. From your arms to your legs to your core, everything about you is power.
This is actually super-helpful for your job. After all, every single pair of eyes in the room will be on you for the entire fifteen-minute to hour-and-a-half class. Every time you jump, they jump; every time you dip, they dip, we dip. It's like a game of Simon Says where Simon always says, and then tells you how you're doing it wrong. If you weren't physically powerful enough to get through your own class, what kind of Simon would you be?
You are more than just physical prowess, though. Much like a politician, you're on a first-name basis with basically everyone you meet. People come to you for advice on workouts, stretching, diet, supplements―even your opinions on the best time of day to do a push-up (answer: depends on your circadian rhythm―you'll know what that is when you become an instructor).
Use this to build your client list and your reputation. If you put enough effort into becoming known as the best Pilates instructor, eventually you'll be dictating how the Pilates calendar should function.
Who knows: Maybe if you run your classes well enough, you might be in line for management, even fitness director or general manager. Take that, tenth grade Phys Ed teacher who thought you were lazy.
Word of warning, though: Gyms, especially membership-only facilities, can be dens of competition, power plays, and excessive machismo. This comes with the territory of working in the same field as people who use the phrase "wailing on my pecs" in everyday conversation.
If you're not careful, you can get swept up in all that—and that's going to have a very detrimental effect on your career. Not everybody gets motivated by having somebody yell at them and belittle them. In fact, most people don't.
You're a fitness instructor, not a fitness dictator. Keep that in mind when you're deciding how to wield your power. How you act will go a long way in determining who your clients are, and how long they want to work with you.