First, a disclaimer. I urge you not to misconstrue what is being suggested here; there is nothing wrong with cardio workouts, and they are in fact necessary for your health. They are a great way to burn calories, keep the circulatory system pumping, and maintain your heart health. Cardiovascular exercise is also a wonderful way to proactively attack stress or have an excuse to watch television or listen to music for an hour while still getting something constructive done.
With that said, if cardio is the only form of exercise your body is getting, you are not helping your metabolism, building muscle, or getting the weight loss benefits of a well-rounded program. What you can accomplish running at a relatively slow and unchallenging, but regular pace is nothing compared to what is possible if you use that same time for a much more varied, structured, and diverse regime.
Cardio work burns major calories but the problem is the benefits mostly end there, at least in terms of weight loss and muscle retention. No matter how many hours you dedicate to the low-impact cardio workout, your metabolic rate is going to stay exactly where it is, thus leading to a plateau. Furthermore, as only muscle burns fat and you are not striving to create new muscle (just burning calories, hour after hour), you are likely to burn muscle instead of fat, and we all know this is no good.
With respect to muscle-burning (versus fat- and calorie-burning) another issue with low-intensity and low-impact cardio workouts is that these long sessions burn many calories but in doing so, they also cause depletion of your fat and glycogen stores. While this may sound good since, after all, you are burning something other than calories, the results can be disastrous to your efforts. Once these stores have been tapped (after your cardio workout has passed the twenty-minute mark), your body is going to look for something more to burn. And guess what the most readily-available source of fuel is? Muscle tissue. This is how aerobic exercise can lead to muscle loss rather than gain, and your goal is to lose weight without losing muscle.
