Historically, fitness was discussed in terms of the capacity to carry out the day's activities without undue fatigue. If you think about the word, this makes perfect sense. The term address how "fit" you are to do a day's labor.

As automation and a more sedentary lifestyle has arisen, this definition has come to make less sense. The industrial revolution meant that one could be quite "fit" for their daily activity without even a modicum of what is now considered fitness. Physical fitness now tends to be discussed in terms of a measure of the body's ability to function efficiently and effectively in work and leisure activities. It also includes the capacity to be healthy, to resist disease and to be able to function in emergency situations.