At age 74, late-bloomer Edwina Brocklesby became the oldest woman in the U.K. to complete an Ironman triathlon.

"I didn't do any exercise at all until I was 50," says Edwina, who was born in 1943. "One day, I went to see an old friend who was running a marathon. I thought that would be fun to do, at least a half marathon, anyway." And so she did.

If you've been sedentary, starting to seriously work out in middle age is a great idea, as Edwina proves. According to a study published in the Journal of Applied Physiology, it helps protect arterial elasticity (reducing the risk of stroke and heart attack), control blood pressure (ditto), and improve cerebral blood flow (keeps you thinking clearly).

And according to the study, people ages 45 to 64 who log at least 10 years of aerobic exercise during those middle years have heart health comparable to healthy young adults, as well as an extended healthy lifespan.

These benefits may come from how exercise makes blood oxygen levels vary. Early studies indicate that variability may lengthen telomeres on the ends of your chromosomes — and your lifespan.

Studies also show that exercise boosts cardiorespiratory fitness (how your heart and lungs pump blood and oxygen). The higher your level of cardiorespiratory fitness, the longer your life.

So aim for five-plus days a week of aerobics (60 minutes daily) plus strength-building exercise at least twice a week for 20 minutes.