Sixty-six year-old here. Have raced and trained consistently for over 50 years, and have no regrets.
Primarily a middle-distance runner/steeple-chaser who also does road and x-country, I wasn't a high mileage runner, although I did do a couple of marathons in my late thirties, I probably haven't gone past 15 miles in one session more than a dozen times in my life.
I did, however train extremely intensely, with either two interval sessions and a race, or three interval sessions per week.
I competed as a club runner in England until my late 30s, then primarily as a road runner in the US since. I placed several times in USATF Masters Championships from ages 46 to 56 (Steeplechase, 5000m, 3000m indoors), and won the World Senior Games 3000m (with due respect to my competitors not the toughest contest) in my age group a couple of years ago, and did place fairly well on the World Masters Rankings in that distance. At that point was doing four EIM sessions (3x2000m, 6x1000m, 12x400m, 16x200m) per week with 3 or 4 mile recovery jogs in between.
No general negative health signs. Facially, I don't think I'm older looking than my age (although that might be because, I'm not out in the sun for much more than an hour at a time. I work from a home-office and am not otherwise outdoors very often), and when I'm fit, I'm a little lighter than when I was running my best 800ms about 40 years ago.
Incidentally, I have a sister who is 64, and has a similar background. She won a world indoor master's title and recently was on some record-breaking master's relay teams. She similarly enjoys good health, and is like an energizer bunny.