The average lifespan of the human population is growing.
Fitness, the ability to do what you want to do, when you want to do it, applies at all ages but becomes paramount in our later years.

This from Science Daily:

Researchers have analyzed the effect of endurance running training on the stiffness index, a variable that is directly related to bone quality. The results confirm that the greater the race distance that is trained, the better; this can be used, therefore, to prevent the progressive decline in bone mineral density that occurs with age. In healthy individuals, bone quality – chiefly determined by bone mineral density – depends on factors such as sex, age, race and diet. It can be modified, however, by making life style changes -for instance by doing regular exercise.

Source: The more you run, the denser your bones will be – ScienceDaily