Can I use apple fitness without an Apple Watch?

If you mean fitness+ then this is what you need;

Apple Fitness+ compatibility.

Apple Fitness+ requires Apple Watch Series 3 or later with watchOS 7.2 or later and one of the following Apple devices: iPhone 6s or later with iOS 14.3 or later, iPad with iPadOS 14.3or later, or Apple TV with tvOS 14.3 or later.