Our first aerobic workout will be moderate in duration: at least 20 minutes and no more than an hour of sustained activity.
But first, we need to gain a better understanding of aerobic exercise. The body requires oxygen in order to breakdown, or metabolize, fat for energy; aerobic literally means, "with oxygen." As long as sufficient oxygen is available, the body can run almost exclusively on fat; this is considered an aerobic state. This makes perfect sense when you consider that higher intensity activity equates to heavier breathing -- your body needs more oxygen in order to burn more fat. At a certain intensity, however, we simply can't take in enough oxygen to keep up with energy output, causing our bodies to switch to an anaerobic state, burning glucose, which is metabolized "without oxygen," as the primary fuel source. This is a normal and vital function that is required for high-intensity exercise and is also precisely what we don't want, during an aerobic workout.
The primary goal of aerobic exercise is to improve the body's ability to efficiently burn fat, which burns cleaner and more efficiently than carbs/glucose (fat metabolism creates less free radical byproducts and therefore less oxidative stress and inflammation). In order to achieve this goal most efficiently, we want to maximize fat metabolism by maintaining an exercise intensity just below the point at which the body needs to switch to glucose; this point is known as the "aerobic threshold" and this type of training is often referred to as "aerobic threshold training."
So how is this accomplished? The most effective way to ensure that our aerobic workouts remain strictly aerobic is by objectively tracking intensity with a heart rate monitor. All aerobic exercise should take place at a heart rate that is between 55-75% of your maximum heart rate (MHR). MHR can be calculated by simply subtracting your age from 220. For a 45-year-old with an MHR of 175, aerobic workouts should take place at a heart rate between 96 and 131. Harder workouts can be done on the upper end of this range, with more mellow "active recovery" type workouts taking place at the lower end. Depending on your current level of fitness, these workouts may start out as anything from a brisk walk to a light jog.
There are a number of health issue that can arise from frequent sustained workouts above one's aerobic threshold, often referred to as "chronic cardio." I will touch on this and the "black hole" heart rate zone later in the week!
Note: if you're already an experienced endurance athlete, aerobic threshold training is going to feel slow at first, but stick with it. You'll be pleasantly surprised how fast your can increase your pace/intensity while remaining in the aerobic zone.