If you especially want to be a fitness instructor, you must study marketing and fitness psychology, and arrange your life after you get old. If you can win prizes in famous competitions and become famous on TV, you have the capital to become an excellent fitness instructor. So that your family will agree with you as a coach. And hobbies will be suitable for changing jobs. This is because if you make money with hobbies, you may have psychological changes and can't keep your heart. If you can keep your initial heart, first take other professional life fitness as an auxiliary business. When you become famous, it will be much smoother to switch to the main business.