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1. Brisk walking

The human body was born to walk. Whether you rack up the miles on a treadmill or hit the road, brisk walking is a natural way to improve your fitness. Wear supportive, comfortable walking shoes, strap on your iPod and get moving. Though a leisure stroll is better than sitting on your couch, push yourself to walk at a fast pace to achieve a moderate intensity level.

2. Running

Though more challenging than walking, running is another heart-healthy physical activity that the human body is ready-made to do (barring physical limitations or injuries). In addition, it is one of the best ways to burn calories (a 150-pound person can burn 100 calories per mile), a bonus if you are also trying to lose weight to reduce your risk of heart disease. If you are a beginner to running, start out with a brisk walk and add 1 to 2 minutes of running every 5 minutes of walking. As you get more fit, you can increase the minutes you run until you don't need to walk in between.

3. Swimming

The pool may be a great place to float lazily along, but that water can also be a full body fitness challenge. Swimming laps or even participating in water fitness classes will not only raise your heart rate and improve your heart health, the water provides multi-directional resistance that will improve your muscular strength and tone. Swimming is a safe alternative if you have joint problems that walking or running can aggravate.

4. Cycling

Another cardiovascular activity that is easy on the joints, cycling is a low-impact exercise that you can do solo in the gym, in a spin class, or outside on the road or trails. Make efficient use of your cycling time and bike to work or to do your errands. Even better, join a cycling club and enjoy the camraderie. While your heart is pumping you'll also be building strength and toning your lower body as well as your core muscles, if you take your bike off-road.

5. Interval or circuit training

If doing a solid stint of aerobic exercise bores you tears, mix up your cardio. For example, for every 3 minutes of cardio, do 1 strength training exercise or a high-intensity burst of cardio for 1 minute. Another option is to choose 5 to 10 strength training exercises and perform 1 set of each, doing lower weight and higher repetitions while moving quickly from one exercise to the next to keep your heart rate up. This type of training will not only keep you motivated to exercise, it will improve your muscular strength, endurance and heart health.