We know we won't live forever. But as long as we are of sound mind and body, most of us would like to stick around for as long as we can. I recently spent an hour on livingto100.com figuring out my life expectancy. While I was sure it would be at least 100, this crystal-ball calculator predicted that at 81, my body would finally fail me. It turns out that every choice we make affects our projected expiration date. Cheat on your diet, and you subtract precious time. But more and more evidence shows that simple tasks like exercising may not only add years to your life but also delay the onset of disabilities in your golden years.

Case in point: Stanford University researchers began studying 538 middle-aged runners back in the 1980s during the height of America's jogging craze. At the time, critics were convinced that runners would suffer serious injuries and predicted an epidemic of knee replacements. But 21 years of research show quite the opposite is true. Data from the Stanford study, which was recently published in two peer-reviewed journals, show that the runners did not have higher rates of osteoarthritis and total knee replacements. And the onset of disabilities appeared 12 to 16 years later in the runners' group vs. the nonrunners'. That's huge; imagine living independently or delaying the use of a cane for an extra decade or more. There were also half as many deaths in the runners' group than in the nonrunners' during the study.

"The longevity effect was big. It surprised us," says Dr. James Fries, co-author of the Stanford study. "But the even bigger difference was morbidity rate — your overall quality of life." Running and other weight-bearing exercises like vigorous walking and using an elliptical machine work to strengthen your bones, tendons and muscles. They also help you reduce your risk of America's biggest killer, heart disease.

The best way to start an exercise regimen is to come up with a goal — such as losing 10 lb., running a half marathon or getting off those blood-pressure meds. Then talk to your doctor to determine your health restraints and map out a plan to achieve your goal. Avoid injuries by investing in a good pair of shoes and by stretching. This may surprise you, but the best time to stretch is after you run, ideally for 15 to 20 min. Muscles respond better to stretching when they're warmed up. Naturally, you'll be sore after beginning an exercise program, but check with a doctor if the pain you experience lasts more than a week. "Studies show running itself isn't bad on the joints," says Dr. Amadeus Mason, an orthopedist at Emory University's Sports Medicine Center. "The issue is if you get an injury and keep running on it."

So don't be afraid to get moving. Just 30 min. of daily exercise will not only add to your quality of life but also prevent diseases and help you live longer — maybe even to 100.

— With reporting by Danielle Dellorto / Atlanta