There are pleeenty of ways you can make money from your fitness app. You can consider freemium upsell, crowdfunding, paid subscription plans, paid premium content, advertisement, sponsorships, selling fitness equipment or other related products (in-app purchase), transaction fees, etc. We’d say sky (your creativity) is the limit here.
Also, if your product is a mobile app, it’s worth making sure it’ll be listed high on the search results in the App Store and Google Play store – as this translates into more downloads = more chances to sell.