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.
You need to work out what your speciality is for example weight loss or body building and then focus on people that want to improve on that. You can start on social media by doing a free 2 week program and when they see great results they will then go on to continue using your online service whilst also telling their friends and family about you