One of the challenges of life is to develop the discipline to always be engaged in behavior or activities that we will to greater long-term satisfaction in life. Sometimes, my kids wonder why I don't eat tons of ice cream, cake, and cookies. Well, yes, I sure like the flavor and I have cravings to eat to my heart's content from time to time. However, I realize that they will stick to me like glue, and they can have long-term effects on are overall health.
For someone who runs a lot in long-distance events, you would think I wouldn't be so concerned about that; yet, we can defy age, and not taking care of our bodies will catch up with us eventually.
It is hard to convince someone in their teens or in their young-adult years of these facts. You can sermonize all you want on these topics, so in the end, the best thing to do is live the best you can and hope your example speaks louder than words.