Fitness encompasses an array of things. Your definition of fitness will vary from the next persons. Here are a few definitions that "fitness" can cover:
- Endurance capacity
- Sprinting ability
As a general rule of thumb, it is important to exercise at a higher level that what you are currently capable of. Otherwise, your body will have no reason to improve your fitness.
This is because improving your fitness requires energy. The body is a naturally lazy organism and does not want to expend calories if it doesn't have to. Therefore, if you do not place sufficient strain upon the body, it's not going to do much in regards to your fitness level.
Exercise is the stimulus to enhance your fitness. Your fitness does not improve during the exercise itself. Rather, your fitness improves as you recover from the physical stress induced from exercise.
Whilst you have to exercise at a level that is more demanding than what you are used to, you do have to ensure that you don't exercise at a level that is "too" demanding. This is often the case when a sedentary person decides to undergo an intense workout. There is far too much of a shock to the body which can cause physical sickness and excess fatigue.
Keep this in mind next time you are training. Doing the same thing over and over again may lead to diminished results and a lot of wasted time and effort!