Given a period of high mileage at some point in a runner's development, I think the chief benefits of "junk miles," or "maintenance runs," are proprioceptive, and metabolic.
They help you with efficiency and keep metabolism stimulates-- keep your weight down.
I'd like to see a 10k time to compare with the road mile.
I'm guessing there would be a huge fall off from what Daniels would predict.
In general, though, older runners definitely benefit from more weight work, more hill work, more recovery, and fewer miles.