running after 50

The way Gretchen Reynolds kicked off her recent New York Times article made me jump for joy - then jump into my jog togs:

[Editor's Note: In the men's division of the 2014 Boston Marathon, run April 21, American Meb Keflezighi, 39, won with a time of 2 hours, 8 minutes and 37 seconds. He was the oldest winner since at least 1930. Rita Jeptoo of Kenya defended her women's title, with a course record of 2:18:57.]

I love to run. I run to clear my head, burn calories, strengthen my lower body . . . and it just feels good.

Okay, so you've made a commitment to exercise more, bought a new pair of running/walking shoes,  gotten the green light from the doctor, have your iPhone (or some other smartphone) strapped around your arm or waist, and watched " Running After 50" from "The Best of Everything" AARP YouTube series…

What's the best work out for people of all ages?