Last April 80 year old Ed Whitlock ran 3:25:43 in the Rotterdam Marathon. In July he ran 5K in 21:32 and 10K in 42:39 at the World Championships for Masters Track and Field in Sacramento. In October he'll be gunning for another age group world record in the Toronto Marathon.
Using the the McMillan Running Calculator, Ed's 10k time predicts a 3:20 marathon finish. My guess is a sub-3 is probably out of reach, but Ed will run somewhere in the 3:15 range.
Ed Whitlock's Five Rules for Marathon Running
5. Personalize your training.
4. Take each run in stride.
3. Run alone.
2. Don't overthink stuff.
1. Enjoy your finish lines.
For more go to Ed Whitlock's still going at 80