I am currently wrestling with whether exercise should be more of a pursuit of overall fitness or of goals. These two are not mutually exclusive but they can sometimes run in opposition. For example, in my opinion, running 16 miles is not as good for you as running 6 miles. 16 overtaxes your muscles, harms your joints and there is diminishing return beyond 30 minutes of exercise to the cardiovascular system. I offer no scientific proof to this statement, just my own physical experience. On the other hand, if you are goal oriented and want to run a marathon at continuously faster times, you must run 16+ miles several times a month.
There are the other issues as well such as mental wellness. It is motivating and uplifting to successfully train for an endurance event. The training gives you a purpose in your life, gives your workouts more focus and tacking difficult assignments is good for the ego. On the other hand, it can be draining on a Friday night to know you have to run 18 miles or bike 60 miles or do both the next morning. Exercise can become a chore, not a religious ceremony.
And then there is vanity. To succeed in endurance events, you must be rail thin. Carrying excessive weight, whether it is muscle or fat, isn't good for half ironman times. But it is good for confidence (the muscle) and attracting the opposite sex. Again, no scientific studies to support this assertion.