In honor of Jack Lalanne, who has just passed away, I have a modest recommendation. Find an exercise routine that works for you, start down the fitness track, and see how far you get. For me, "a program that works for you" means a workout routine that you enjoy. There are so many options today, ranging from the Wii Fit and aerobics to running and weight training. But to begin with, get yourself an exercise book. I prefer a comprehensive workout guide produced by Men's Health. There are an equal number (I imagine, I haven't done a survey) of guides for women and men. However, choose carefully. Choose a guide that is practical, that provides workout routines that may be performed without any expensive equipment (showing options for using filled milk jugs as well as hand weights and your home staircase as well as a StairMaster). Further, the guide you choose should advise you to exercise for your health and not for a sculpted body. Let's face it, if you're no longer in your teens or twenties, your chance to look like an Olympic athlete has passed you by. Further, if you don't have all day, every day to work out, you'll never attain that form. The Men's Health guide gives a more practical spin. They state, if you want to be able to carry your grandchildren up the stairs to bed with the same ease you carried your own children, work out for your health. Now, that's practical advice. Make sure to get two copies of this book if you have an active teen or two at home ... you're going to lose the first copy to a younger, avid exercise fan.
My personal choice is weightlifting. The equipment is always ready and requires no electricity. I choose to exercise with a simple weight set and bench in my own home. Why? Because I'm not tempted to do something stupid when working out alone. I keep the weights light to protect myself from injury and increase repetitions regularly to make up the difference. There is no sense in heaving the heaviest weight you can only to blow out a muscle mass and lose weeks or months hobbling around before you can exercise again. Further, especially if you are out of your teen years, never, never, ever try to lift the heaviest weight you can in one try. This is great for the ego and terrible for the body. You stop breathing on such a lift and your blood pressure skyrockets. You are risking a massive stroke every time you try this. It is not worth the risk.
Oh yes, one piece of advice. If you find there is one particular exercise that you HATE (not because it hurts you--stop that one--but because it pushes your muscles hard) ... keep it up. I have found you hate it because you need it most.
So, take Jack's advice, exercise, and feel better. It'll reduce the number of aspirin you take, reduce aches and pains (if done properly), and give you more energy. Most of all, once begun, if done regularly, your body will soon ask for more. Hard to believe now, but it's true.