A fitness site can have numerous revenue streams. Create healthy eating plans and recipes and lock them away as premium content. Promote and sell a fitness training course, eBook, or a series of videos. Or advertise your own personal fitness services on your site. Topfithub is a good example of a fitness site with decent video content alongside product reviews.
How much will you make? I think my best month with Google AdSense was almost $5,000 over the last ten years. That amazing month blew my mind since it was actually near the beginning of my blogging journey. When you go from making zero to $5,000 in a month, that will rock your world. For me, it also got me even more excited because I knew there were other ways to monetize.