The summer before my junior year in high school, I did about six weeks averaging over 100 miles a week. However, I only managed to run a marathon that fall in 3:23.
The summer before my senior year in high school, I “only” ran about 70 to 80 miles a week, but emphasize speed work intervals, tempo runs, farlek, etc. That fall, I ran a 2:59 Marathon.
However, I believe that I would’ve never done a 2:59, if I not been doing 100 miles a week the year before
I’m in my 50s now, and one thing I noticed over the last 40 years is that the more miles I run a week, the faster I can run a marathon.
It seems to me, that you just have to put the miles in and there’s no short cuts