16 of the top 26 US marathoners ran between 1972 and 1989. I would have counted Kempanian but I didn't want to get greedy but he did train like the 80's guys. Puskedra just cracked the list to push Shorters 1972 time off the list , all ran 2:10:30 or better. Go down to 6 -11 and my argument gets stronger. I am looking for an overall improvement not a handful of outliers.
6. Alberto Salazar—2:08:52, Boston Marathon, 1982
7. Dick Beardsley—2:08:54, Boston Marathon, 1982
8. Abdi Abdirahman—2:08:56, Chicago Marathon, 2006
9. Greg Meyer—2:09:00, Boston Marathon, 1983
10. Bill Rodgers—2:09:27, Boston Marathon, 1979
11. Ron Tabb—2:09:31, Boston Marathon, 1983
Read more at
http://running.competitor.com/2015/05/news/top-25-all-time-u-s-marathon-times-men_129010#pf0hurG5fhEIdxOY.99