There are lots of great reasons to exercise, and staying heart healthy is just one of them. Regular aerobic exercise can lower your LDL (bad cholesterol) and blood pressure as well as your risk of obesity, type 2 diabetes and other conditions. It also reduces your stress level. Your heart is a muscle that needs to be exercised just like any other -- doing so will make it work more efficiently even when you're at rest.
The American Heart Association suggests that you get 30 minutes of exercise on most days of the week. If you've never exercised before, start out slowly and build up. You can also break up that 30 minutes into two or three sessions. Jogging, hiking, brisk walking, swimming and bicycling are all good places to start, but finding something that you like doing (and will stick with) is the most important part.