Many of us like to believe we get "enough" exercise in our daily lives without dedicating some time specifically to working out. In most cases this is roughly equivalent to someone who eats only ice cream for breakfast, lunch and dinner saying they get enough food each day! The reality is that we are looking at just quantity, which is meaningless without considering the type and quality of the exercise. The person who only eats ice cream may get sufficient calories, but they will be lacking many necessary nutrients. In the same way, the person who gets "enough" exercise is also typically lacking the right kind of exercise.
Our bodies are built to adapt and survive in a wide variety of extreme environments, and one of the ways they do this is to shed any "unnecessary" aspects such as unused muscle, excess bone mass, etc. Any muscle we don't use is discarded (often referred to as "atrophy"). If we simply underutilize it, the body discards part of it. The same is true of bone - if it is not stressed our bone density drops. Because of this, it is not sufficient to just get "enough" exercise, we must get exercise which stresses each of our muscles and bones to a point close to it's current limits (both in speed and strength) on a regular basis in order to simply maintain our bodies. If we only work part of our bodies, we only maintain part of them. Many of the symptoms we associate with aging have little or nothing to do with age, and are, in fact, simply the result of atrophy. You could, in fact, develop these problems at any age by lying in bed for months, being put into a full body cast, or more slowly by simply leading a very sedentary life.
Some of the many ailments which have been shown to (at least in part) be either directly caused by or which have a high level of correlation with lack of exercise include:
A complete list would actually go on for many pages, and you would likely recognize a large number of them. So if you think you get "enough" exercise in your daily life, you might want to re-think it.