I’m 62. Haven’t done it yet...we’ll see.
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.