Everything is impermanent. To keep something the same and redirect its natural tendency to change, we must maintain it. (Rust never sleeps) To advance or improve, we must apply consistent effort (intensity) regularly (frequency) for a sufficient period of time (duration).
For most of us in youth, we are introduced to physical fitness by participating in sports. Sports are basically competitive athleticism. We train to obtain greater aerobic endurance, stronger muscles to overcome resistance, and refined skills required by a particular sport. We gauge our success by judging it against other.
Being fit is different than being healthy. Being in good health is determined by a doctor using a series of medical tests. The exercises to keep a person healthy do not need to be as arduous as those required to reach heights of athletic superiority. It is possible to be fit and not healthy. For example, the injuries professional football players collect over the course of their career can have lifetime implications (concussions). Many long term Karate and Kung Fu practitioners require hip and knee replacement surgery due to the relentless ballistic stresses upon those joints.
We need to acknowledge the fact, that as we age our bodies are in retrograde. (Generally after 30, the body begins to degrade because on a cellular level, cells cannot reproduce perfectly to 100%) Forced improper body usage during exercise or the wrong combination of intensity, frequency and duration can lead to failure to remain healthy or actually accelerate the aging process.
The point is, what is appropriate for youth may not be appropriate for the mature adult. Although sports can be an excellent form of a health maintenance, injuries can easily occur derailing and setting back actualized gains in fitness. No pain, no gain is not therapeutic. We should assess our own health and practice at a level that is sustainable to reach our goals. Health exercise is not a competition with others.
By setting realistic short term goals and fostering a long term perspective is a good plan to remain healthy throughout life. Tai Chi engagement is to harmonizing with nature. Applying the principle of Wu Wei (non doing) is to push the canoe with the current and take advantage of the natural flow of life. To be constantly at odds with life and battling to hold the position of superiority is not in accordance with Dao.