Last but not the least is the nutritional component. It's not just knowing what to eat but it's knowing how much of it to eat and when to eat it.
This does take some work. It also takes trial and error because any changes you make in your nutritional habits are not going to show immediately. Sometimes they'll show up two or three weeks later. Consistency when making changes will help you decide whether or not the change was a good one or a bad one.
Then you have the ageing process, which continues to try to change the body to make it old and to slow your metabolism down. Once you've figured it out, you've only figured it out for the moment. Sooner or later you're going to have to make another change. And that's the story very simply put.
In summation, the mental, physical, and nutritional factors influence your health and fitness. All are necessary equally.