The average marathoner doesn't run 75 miles in TWO weeks!
I used to be one of the local fast guys, running in the low 2:40s. At that time, I'd top out at about 80 mpw, although the average was probably 60-70. Now, some 2 decades later, I run over a minute a mile slower than that, and I top out at about 40 mpw. Believe me, there are lots of marathoners doing less.
When you're running 75 mpw, you don't even realize how out of touch you are with what average really is.
If you could somehow measure, I'd bet the average is closer to 30 than 50.