Pursue fitness because it's a lagging indicator

Fitness is one of the single-most beneficial traits an individual can commit to building and maintaining because pursuing fitness leads to deliberate, long-term choices that have secondary effects of happiness, health, and longevity. Overall, the trait is a lagging indicator of your diet, exercise, and sleep habits, so by pursuing a fitness goal, you are committing to a number of positive behaviors involving those habits and have to stick to them for a while.

Sleep is a big one--sleep always catches up to you if you don't get enough, but without regular workouts, you can get away with an irregular or night-owl like schedule. If you are working out in a manner that builds or maintains fitness (so not counting walking as exercise for an able-bodied adult, for example), you will very quickly be driven to a fatigue that forces you to change something. Either you will stop working out and maintain your irregular, late-night sleep schedule, or you will adopt a consistent, early or not-too-late sleep schedule. "Early to bed, early to rise..." Sleep is important for mood, physical performance, and intelligence--really, just about everything.

You may pick up all manner of exercise routines to meet your fitness goals, from cycling to weightlifting to yoga. You will also need to be consistent with your exercise and potentially deliberate in how you increase or decrease the amount and intensity. Exercising for exercising's sake--because you enjoy it or because you know a certain activity is helpful for you--is always a good thing, but you will not achieve the same benefits as from the consistency and deliberation that comes from exercising as part of a broader fitness goal. Generally, also, whatever levels of exercise lead to improved performance should also lead to improved health. Endurance athletes have lengthened telomeres, which drive cancer rates down and lead to slower aging for you and longevity of your offspring, and individuals who consistently lift weights or do cardio exercises have anywhere from a 20% to 40% reduction in mortality.

Then, diet improves because you have to consider what outcomes you want and what nutrition is needed to get there, whereas we might normally tend to eat whatever happens to be nearby. You will likely find that drinking alcohol, eating sugar, and eating processed foods in excess, along with other blatantly bad nutrition options, will thwart your fitness goals around either body image or some performance outcome. Although we have no idea what food and diet lead to longevity, I'll pose the idea that whatever food and diet hamper your fitness goals are probably also impacting your health and longevity negatively.

We naturally follow the herd and let our environment dictate our choices when we don't put thought to them. Pursuing fitness forces you to reconsider your choices and overcome any negative influences from the herd or the environment you find yourself in. Pick a fun physical activity or sport and identify a related goal around performance or body-image, and you'll quickly find that you are forced to re-assess your habits around diet, exercise, and sleep. You should also find that the new habits you settle on will lead to a happier, healthier, and longer life--but only after you stick to them for a while, since it's a lagging indicator after all.