I just read an interview with Hill who says he topped out at 156 and found it wasn't doing anything for him so he didn't stay that high. I had a similar experience on a much lesser scale.
I always followed the school year and used summer as the time to run big miles. That meant different things at different stages of my life ranging from 50-60 when I was fairly young to 120-150 in my prime. I never worried much about pace when I did it. The weather was generally hot and it was very hilly where I lived. But I had time, especially when I was teaching and didn't work in the summer, and the days were longer. I usually didn't like running in the dark.
Come fall, with a return to school, less time, cooler weather and more races, I usually lowered the miles, say to 100-115 and generally more on the lower end. The combination of fewer miles and cooler weather always brought the pace down. As the months went on I'd get stronger and more adapted to that particular pace. I'd have a mini build-up around Christmas, New Year and early January when I'd get to 120 or so at times, but this time I'd usually be running at the faster pace from the fall. When the next summer came I usually was able to maintain that pace for the bigger miles in hotter weather. Come the following autumn I'd repeat the pattern and drop the miles and quicken the pace.
This all came naturally. There was no conscious attempt at lowering the pace each fall or at maintaining that faster pace into the next summer. It just happened. The effort was pretty much staedy for years and years. This worked pretty well from 1971 to 1975. 1976 was my biggest year mileage wise. I had more weeks over 140 than ever, got to and over 150 for the first time ever, but didn't seem to have the natural speed up come the fall. In 1977 I didn't push the miles as high but added regular runs of 25-30 miles. Same thing, no real bounce into the fall and no new PR in either 1976 or 1977.
It's hard to be precise about something like this that happened over thirty years ago but what I concluded from all of this was that going over 140 consistently and later doing really long runs frequently must have pushed me past the point where I was recovering enough to get the maximum training effect. One thing that doing the 150 weeks does do is make the 130 weeks seem relatively easy though.
You never know what your mileage maximum is until you've exceeded it and I think I did that. If I could go back in time and advise myself I'd tell myself to stop at 135-140. But I only know to say that because I didn't.