At age 58, read a story about a lady who was 60 and started training for the "Ride the Rockies" (about 400 miles of Colorado passes).
I figured if she could do it, so could I.
Now, I had ridden before, but only casually and like 5 mile rides. Within 3 months I had 1100 miles on my bike. An I did RTR two years in a row.
Now up to over 15,000 miles and still pedaling away!