If you have the capital, advertise your product or page on the big fitness accounts (500k+), do your research on a decent price, and if you product is of quality value, you'll get a decent roi
If you don't have the budget, do a lot of shoutout for shoutouts with account fitness accounts that are a similar size to yours.
But do a at least 1 post a day advertising your product. Maybe even start an email list with a free ebook you can write, then promise to send a fitness related article to them once a week and plug your product long term.
I hope this helps you brainstorm some ideas