One of my best friend used to own of Canada's largest mobile fitness franchisee so I feel like I can answer your question quite well.
The first thing you need to focus on is your CAC (Customer acquisition cost).
How much marketing does it cost to get 1 client. Once you have a decent idea of your CAC you can start scaling your fitness courses.
The best possible way to do it is by running webinar sessions targeting a specific fitness problem and then offering your course as a solution with some bonus materials at the end of the webinar.