I agree. At some point your improvement will not be much at all
as the miles get tacked on. Sometimes the more miles that get tacked on the worse you get. Alberto Salazar got worse when he went from 100 to 120 miles per week. If the gold medal was awarded to who ran the most, then there would be guys trying to run 500 mile weeks.
I personally prefer about 70-80 miles per week on singles. I don't know if I would see any benefit by tacking on some filler miles in the morning and putting the weekly mileage to 110 miles per week. I am guessing I would not show any significant improvement for this extra effort and I would probably come down with an injury. Just a guess though.
One thing is for certain, a lot of people have beat me by only running 40 miles per week. My teamate of mine ran 14:20 5000m off of 40 miles per week. He was very light and was able to handle the massive amounts of intervals where I was more of a stocky runner and broke down from the intense workouts.