The concept of fitness is sometimes confusing and we may even revise this relatively simple conceptualization. Many theorists view it through their own prism and take their specific interests or personal conceptualization and raise one component of fitness to the highest rank of importance. They then either neglect or even belittle the other facets of fitness. We prefer a balanced take on fitness and view fitness as having three pillars. (If the pillar thing is too large to get a handle on, you can look at it as a stool with three legs which work best in balance.)
While portions of the system overlap, this system gives a nice, simple overview of the areas of focus for a fitness program. An example of overlap and cross over would include a concept of “bone fitness” as crossing between musculoskeletal and physiological.
My view of the pillars of fitness include:
Cardiovascular
Musculoskeletal
Physiological
Cardiovascular – Heart, lungs, circulation all are parts of the cardiovascular system. Aerobic fitness is most often what we are referring to, but we can include all aspects within this system. The formula for maximum oxygen uptake takes into account the amount of oxygenated blood that can be delivered to the muscle “end organs”, and the ability of the oxygen to be utilized by the muscle. Aerobic exercise is the means by which this system is usually improved. But interval training certainly is helpful. Interval training does not mean going all out. It means spending some time outside of your zone of comfort and going a bit faster or harder than you normally do.
Musculoskeletal Fitness – Musculoskeletal fitness includes your muscular system and your bones. Strength training, also called resistance training is the key which needs to be added into your fitness program. Strength training enhances muscular function, aids body composition, lowers body fat, can gently stress the skeletal system to help prevent or slow most especially spine osteoporosis and other areas of osteopenia or osteoporosis. Having stronger muscles helps prevent, treat, or lessen the impact of sarcopenia, which is muscle loss that occurs as we age. Having stronger muscles, which react well, can help improve balance, and may help prevent falls which cause bone injury, including hip fractures. This should also be a component of a life long fitness plan, and luckily has been becoming increasingly popular. Exercises can be done with machines, free weights, and even a complete body weight workout is possible.
Physiological – Physiological fitness comes about by evaluating your blood chemisty and body makeup and then acting upon it. Blood lipids, HDLs, LDLs, Cholesterol are the most well known parts of the blood chemistry to be evaluated. While there is a very minor controversy about the impact of total cholesterol on heart disease and strokes and the relationship is incompletely understood, there is a general consensus by most physicians that it is not a good thing to have a high total cholesterol. The research on that seems to me to be relatively clear. The research on LDL, triglycerides, etc. is a little less clear. But regarding cholesterol, the consensus is clear that it is not good to have a cholesterol over 200 and clearly 280+ is not good at all. Saying otherwise is an outlier position, it is analogous to saying that cigarettes have no relationship to lung cancer. You can consider first trying diet and exercise, but you may need medication. Be aware of the side-effects of statins and other medications, and if you have concerns, discuss them with your medical doctor. Do not take comfort in an outlier position, no matter how much other good advice the outlier may provide, this part of medicine, as accepted as it is, is about as far as we can go at the current time. In 10-20 more years, the advice will be more specific, and those at risk will be more easily separated from those who are not at risk. But again, this is about as good as we can predict at the moment.
Body composition and body fat is another area that would fall under physiological fitness. Diet evaluation and modifications, exercise, caloric balance, and 
where needed medication can help this area of fitness.
These are the areas to think about as you set up your fitness program. Don’t neglect any of them. Say no to being short sighted and blinded to a comprehensive fitness program. Do it all! But start out slowly and carefully, and find a good program and certified trainer to help.