Here is a definition of the word 'fitness'
fit·ness
n.
1. The state or condition of being physically sound and healthy, especially as the result of exercise and proper nutrition.

Some of the health benefits of being fit are:

• Improved ability to fall asleep and sleep well;
• Reduced feelings of depression and anxiety;
• Increased energy;
• Increased mental acuity — sharper and faster thinking.
• A healthier heart;
• Healthy muscles, bones, and joints;
• Increased burning of calories;
• Better ability to cope with stress;

Being fit also reduces the risk of:

• Developing type 2 diabetes;
• Developing colon and breast cancer;
• Becoming obese;
• Dying early;
• Coronary artery disease;
• Developing high blood pressure;
• Depression