A good customer experience goes a long way in delighting your users, keeping them engaged and using your product. There are many off the shelf CRM products out there, depending on the core features/service offering of your fitness product you can pick the one that best suits your needs from capabilities and price.

In regard to improving the user experience you may need to focus on a few key areas while gathering insights to improve the product /UX - user feedback, quantitative/qualitative research, analytics and iterative design. Many off these tools can also be used to support a mobile user experience if that is also part of your product offering.

I have designed & launched full lifecycle products web & mobile - from concept to launch leveraging different platforms. I would be happy to get on a call & discuss your project so I can provide detailed direction