I know someone who didn’t run, at least as he tells me, until he was in his sixties, maybe even mid; now, a few years later, he won his age class in a major marathon. I think it was along the lines of 4 hours and 23 minutes but I could be wrong.
Honestly, he’s about 5’8”, maybe 5’6” and rather lean, wiry, probably less than 150 lbs. I think that is as crucial in his case as anything. A lot of us don’t carry much extra weight but if one is big and say, of a medium build, I think it makes it harder.
I’d love to get motivated to do that.
One of my coworkers picked it up again in his 50’s after having not done it since HS. We’d hired a woman about 20 years younger than him that did ultras and they hit it off. They both do ultras now and are married :-)