I don't know. If you look at a list of fastest U.S. men's marathoners, there are an awful lot of times from the 1970's and 1980's -- when there wasn't a lot of money in the sport. I think it's just a different mindset. Love him or hate him, Rupp is the only one who goes in thinking he can run with the best.
This is a very good point. I can't pretend to know about much about that era as I do present time, but the running boom came with big time mileage too. I guess that part is all relative once you're above 100 mpw during a training block. Ryan Hall noted his biggest regret was not focusing more on the 5/10k speed once he made his jump to the marathon. Rupp was a few months shy of 30 before his debut, and Mo Farah (I don't believe he ran one) before the age of 35, so idk if running you're first marathon at 27-30 is the worst thing for an athlete either. It's raises an interesting question though as we're seeing more Americans struggling to crack the 2:10 barrier. If an athlete is running sub 28 or low 28s at 22-23 y/o, do they make the marathon jump by 25-26? Or does it all depend what sponsorship opportunities coincide with the marathon cycle?