What is the best fat-burning exercise for a 48-year-old woman?

Anything that uses the large muscles and as many muscle groups as possible. Walking, running, raquetball, tennis, cycling or whatever YOU find enjoyable. You need to maintain "aerobic" status, meaning that you shouldn't get out of breath while you're exercising, but you need to exercise as vigorously as possible. Exercise for at least 30 to 45 minutes 3 times per week. Anything else is a bonus.