If you had time to run doubles 3-4 times a week, would you? Or is generally not smart to run doubles (high mileage) as an older runner? If not, would the extra time be better used to hit the weights, get extra sleep or anything else but run??
It seems that only a certain type of older runner can run higher mileage (75+ mpw). Even when splitting easy runs into shorter AM/PM runs it sometimes seems for me that might not be enough.