Experts recommend that you do some form of aerobic exercise at least three times a week for a minimum of 20 continuous minutes. Of course, if that is too much, start with a shorter time span and gradually build up to the minimum. Then gradually progress until you are able to work aerobically for 20-40 minutes. If you need to lose a large amount of weight, you may want to do your aerobic workout five times a week.
It is important to exercise at an intensity vigorous enough to cause your heart rate and breathing to increase. How hard you should exercise depends to a certain degree on your age, and is determined by measuring your heart rate in beats per minute.
The heart rate you should maintain is called your target heart rate, and there are several ways you can arrive at this figure. The simplest is to subtract your age from 220 and then calculate 60 to 80% of that figure. Beginners should maintain the 60 percent level, more advanced can work up to the 80 percent level. This is just a guide however, and people with any medical limitations should discuss this formula with their physician.
You can do different types of aerobic activities, say walking one day, riding a bike the next. Make sure you choose an activity that can be done regularly, and is enjoyable for you. The important thing to remember is not to skip too many days between workouts or fitness benefits will be lost. If you must lose a few days, gradually work back into your routine.