To continue the comparison with eating, most people know we should eat every day (frequency), that it's better to eat the proper amounts of the right kinds of food (duration), and also that it's best to have several (3-5) small meals during the day rather than one huge meal (intensity). The difference between intensity and duration may be a bit fuzzy with the food comparison, so let's switch right off to exercise.
Frequency - How often you exercise is the most important of the three factors. Bottom line, you should exercise every day. You don't need to do every type of exercise every day, but every day you really should have a plan to include some form of exercise. Certainly cardiovascular exercise and stretching are good every day. Strength-building with the same muscle groups isn't a good idea because doesn't allow time for muscles to rest, recover, and grow. If you exercise every day, no matter how long or how hard, that's the first and most important step.
Duration - Once you're in the life habit of working out every day, even if that means a 20 minute brisk walk on some days, duration is the second most important principle. How long you work out depends on many factors, certainly including other things in your life like work, family, social obligations, and rest, but for many it's too easy to skimp. Certainly 5 minutes of cardio every day is better than nothing every day, but after a short while the benefit is minimal.
So figure out how long you need to exercise and make it a priority. Unless you're a competitive athlete, whose long, frequent, hard workouts are the rule rather than the exception, you'll likely find that as you get older, the optimum recommended amount of daily exercise increases. When I was in my 30s and 40s, an hour workout a day seemed sufficient. Now that I'm older, two hours a day just seems right for me, split into one long workout and one or two shorter sessions (remembering that as I'm defining it, a workout can be a brisk walk, not always something intense like Insanity or P90X).
Intensity - The third and certainly not the least principle in exercise planning is intensity. To continue to get benefit from exercise, as you get stronger and more fit, you have to increase the intensity, otherwise you will start to backslide - no joke, that's just the way it is. As your bodies accommodates new levels of intensity, the benefits diminish.
You also need to be moderate with increases in intensity, however, because increasing too much can be harmful. Weekend warrior-type injuries and both soft tissue (muscles, tendons, and ligaments) and hard tissue (bone) injuries can results from doing too much too soon, especially if intensity increases aren't accompanied by increase in frequency and duration.
The Sensible Combination - So the smartest, healthiest, and best combination of these three exercise principles for long term health and fitness is firrst to schedule work outs every day, next to increase the duration, and finally to gradually increase intensity. Simple, right?
