You'll have a hard time enjoying your retirement if you're in poor health. Life expectancy for 65-year-olds in the United States and Europe is well over the age of 80, so you can anticipate living for at least 20 years in retirement, and more if you retire early. Those will be wasted years if you're constrained by chronic bad health.
There are three basic components to good health. The first is a proper diet. It's no secret that obesity has become epidemic worldwide. We eat larger portions of food that's not good for us. Spend a Sunday morning at an International House of Pancakes (IHOP), order a reasonable omelet and perhaps a side of bacon with the beverage of your choice, and observe the people around you. Many will have large platters with a stack of pancakes smothered with syrup, a few different breakfast meats, and some eggs or fried potatoes on the side. At least half of these people will be obese. A "Grand Slam Breakfast" indeed. There's no way we need to eat that much food in a single sitting! If you eat a reasonable portion of food slowly, your body will have ample time to tell your brain when you're no longer hungry. Stick with lean meats, plenty of green vegetables (raw or steamed), and small portions of potatoes or some other carbohydrate. You don't have to shun sweets, but don't overindulge. If you're eating a large but delicious piece of cake and find that you're quite full after eating half of it, for instance, don't force down the other half; stow the leftover in piece of Tupperware and finish it tomorrow! And, for a snack, instead of chips, have a piece of fruit or a carrot.
If you're overweight and want to lose, steer clear of quick weight-loss diets; you may lose considerably over the course of a month or two, but you're likely to gain it all back and more when you go off the diet. Rather, ease into a reasonable diet, as above, and lose the weight gradually. By the time you're down to a weight you like, you'll be accustomed to a different way of eating and will stay slim.
The second component to good health is exercise. Traditional work done by people involved plenty of exercise already -- outdoor labor, heavy factory work, work that entailed moving around. These days, we're just as likely to spend our careers behind a desk and need to make a special effort to exercise. You should exercise at least four times a week, and each session should include some cardio (running, swimming, cycling), which strengthens your heart, circulation, and lung power; weight training (weight lifting or using a resistance machine), which tones your muscles and builds stamina; and stretching, which keeps you limber and less likely to hurt yourself if you fall down. Many trainers also recommend balance exercises for seniors.
Finally, observe good living habits. Don't smoke cigarettes (or anything else for that matter!). Don't overmedicate yourself. Don't drink excessively. Get enough sleep -- most people need 8 hours a night. Do whatever you can to reduce stress in your life -- if you've been given too much to do at the office, learn to delegate or to just say "no." Find enough time to relax and do what you want to do -- avoid bringing work home. And many people find that a "spiritual life" can be beneficial to one's overall health. "Spirituality" means something different to every individual, but if you find that you're just not satisfied with how you're leading your life, then think about some major changes.
The human body is amazingly resilient, and it's never too late to start out on a good health regime. Whatever you've done wrong, you can probably undo. But the earlier you get started, the more years of enjoyment you'll have.