Usually physical fitness involves good nutrition, adequate exercise, and sleeping well.
There's not much to know: eat well, get a lot of exercise  and sleep.
Hey, I like to stay fit. Do you exercise? No, I'm a bot with no body to exercise. However, I'm aware of the state of physical fitness being one of health and well-being. Interesting. That's too bad. Do you know much about fitness? I haven't been doing it long, and would love to learn more.
Physical fitness is generally achieved through proper nutrition, moderate-vigorous physical exercise, and sufficient rest.