I think of fitness as a stool which needs three "legs" to keep it upright and in balance.
The first of these is diet. A diet that is too high in calories or fat is obviously unhealthy, as is a diet lacking in vitamins, minerals, or sufficient fiber. I don't obsess over my diet, but I do make it a point to try and eat as healthily as I reasonably can. I know I feel, look, and perform a lot better when I do.
The second of these is exercise. I try and keep a balance between resistance and aerobic exercise. At a minimum I try and take at least three sessions of aerobic exercise that raise my heartrate to the mid-150 bpm range, for forty minutes or more, each week. I periodically use interval-type training to improve heart-lung function. On the resistance side, I try to exercise each major muscle group once per week (any more and you ARE overtraining). I've recently started paying more attention to my "core" muscles, which has paid some surprising benefits - ie. I'm starting to develop a noticeable "six-pack" at age 48. Whatever your age, gender, or physical condition - you need to have at least SOME resistance exercise.
The last is sleep. I cannot overemphasize how important it is to get adequate rest. Without enough sleep we find our mental and physical performance is hopelessly compromised. Sleep is time when our bodies and minds repair themselves from the damage and stress that day-to-day life imposes on us. I've found that practicing a regime of good "sleep hygiene" (ie. limiting caffeine and alcohol in the evening, and getting proper diet and exercise, along with a well thought-out sleeping environment - clean sheets, comfortable supportive bed, and NO TV in the bedroom) has paid tremendous dividends.
These three factors as, IMHO, the most important in not only keeping fit, but also in enjoying a pleasant and enjoyable life.