If you notice that you don't exercise enough and think: I'm just not much of an athlete, then that reinforces the behaviour. If you really want to change something in the long term, you
will not only have to change your thoughts, but also your behaviour.
The experience I have with my clients who consciously engage in healthy eating and more exercise for two weeks quickly see concrete results. The self-image then changes to: I can muster the discipline to take care of myself. Such successful experiences greatly strengthen your motivation, more than just an adjustment of thoughts, and ensure long-lasting change.
A healthy mindset is reflected in behavior. After all, what you give attention, grows!