Exercise is beneficial to the human body in many different ways and on many different levels. The beautiful part about beneficial exercise is that it does not have to be intense or boring, or even nonproductive. Mowing the lawn is beneficial exercise, and so is scrubbing the kitchen floor. As long as you are exerting energy at a level high enough to increase your heart rate and your breathing rate, the exercise is beneficial. Walking, swimming, and biking are all beneficial exercises that can be fun.
If you have been sedentary for awhile, it is very, very wise to check with your doctor before you begin even a moderate exercise program. It is also important to remember that you didn't get OUT of shape overnight, and you won't get back IN shape overnight, either.
Start slow and add to your exercise program slowly but steadily. For example, you might start by walking around the running track at the local high school one time and at a slow rate. Then you increase the rate a little. Then you increase the distance a little. You keep making little increases over a few weeks, and the next thing you know, you will be walking at a good clip four times around the running track in about 20 minutes. That is aerobic exercise.
Exercise (while you are doing it) actually increases blood pressure and heart rate. However, after you stop exercising, your blood pressure and you heart rate will be lower and slower than before you began to exercise. It is a good idea to take your blood pressure and your heart rate just before you exercise, again when you are through exercising, and then once more in about a half hour. You will see measurable results almost immediately.