I personally know two 45+ year old guys who up until very recently ran something like 65+ races a year, usually doubling and sometimes tripling on the weekend. My observation is that they really didn't run all that much - rarely, if ever, doing long runs and they still did one or two "hard" and fast workouts during the week. They also seemed to take days off weekly and so had pretty low mileage.
They are about as different physically as can be - one very skinny, one pretty muscular. The only constants were low mileage, often with a day off on Friday or Monday, fast workouts T/Th and two races on weekend. It probably helped that neither had high stress jobs or lots of family commitments.
Age is catching up with them, though, as both were recently injured after years of racing like that.