Set up a business bank account, pay you, and deposit all of the funds into the business bank account. Pay all of your fitness related expenses out of the same account. File a schedule C on your tax return (independent contractor income / expenses). Drawback: As a sole proprietor without a corporate structure, if anyone gets hurt and they sue you, they can sue you personally, for everything you own.
Incorporate an LLC (limited liability company) and do your transactions from within the LLC. It takes some expense and effort to set up, and as a separate legal entity it needs its own tax return and the shareholder (you) gets a K-1 to include the income on you rown return. So there’s some work to it. But, if you get sued, all they can take are the assets of the corporation.
I’m not a lawyer, and this is not intended to be individual legal advice. I am, however, a former tax accountant so that part I’m very confident about!