For optimal cardiovascular health and fitness benefits, your ultimate goal should be to exercise aerobically for 60 minutes, at least four or five days a week. You should work up to this goal gradually, however, over a period of eight to 10 to 12 weeks. This approach will give your tendons and bones a chance to adjust to the new workload, while at the same time gradually raising the fitness level of your heart and working muscles. Increasing the intensity or volume of a new exercise too rapidly is the main cause of injury, so go slowly and progress cautiously. If you are older than 40 or have had medical problems that could be worsened by exercise (for example, high blood pressure, heart disease or asthma), it is best to check with your doctor before beginning a new exercise program.
You can begin structuring your aerobic workout. Start with low intensity 20-minute sessions and then add five minutes to your workout every one to two weeks. By week nine, you'll have reached your goal of 45 minutes per workout.
Every workout should begin with five minutes of very easy movement, such as slow walking or cycling or, for swimmers, an easy breaststroke. The workout should also end with another five minutes of the same easy movement. This approach gives your body a chance to adapt to the higher cardiovascular demands of the workout and then slowly return to a lower work state when the workout is completed.
The rest of the workout should be done at a steady, comfortable pace fast enough to slightly elevate your breathing, but not so fast that you find yourself panting for breath. As you become more fit, you may find yourself naturally moving at a somewhat faster pace than before. Occasionally, you may also want to pick up the pace slightly for several minutes at a time, to accelerate your cardiovascular improvement. Use the conversation rule to avoid going too fast: You should always be fresh enough to carry on a conversation with a training partner. If you're too winded to talk, you're exceeding your aerobic limit.
It is best to stretch five to 10 minutes into your warm-up. Extended stretching should be done at the end of the workout, immediately after your cool-down.