The late science fiction writer Isaac Asimov sat at his manual typewriter and wrote books for six whole hours every morning. He typed when he felt like it, and he typed when he didn’t feel like it. Asimov published 400 books in his lifetime using this simple method.
Getting your body in shape works the same way.
First, you decide you’re going to be fit.
Second, you do what a fit person would do: Show up to the gym every day and think carefully about what you put in your body.