I've run 3 marathons, all after the age of 40, and I've known women, many older who continue to inspire me with physical activity.
AARP Bulletin this morning, 8-15 has a video clip of the oldest woman marathoner in the country, Margaret Hagerty, 85 who has run 80 marathons, all after 64 years of age!
Check out her video at http://bulletin.aarp.org/ Scroll down the page a little to the multimedia specials. Sorry I can't add it here, not tech savvy enough.