Found this book in a storage box, took a while to find it...From Hal Higdon's "Boston: A Century of Running":
1975-1979, Men's qualifying time, 3:00.
In 1980, That was dropped to 2:50.
Kerry's "Marathon Experience" took place at the peak of the running boom. The standards from 1970 on got tougher and tougher, only to be gently relaxed, make that very gently relaxed, since.
I find that hard to believe that that standard applied to all runners regardless of age or sex. Women were allowed to run in the race in 1972. Nina Kissick won with a time of 3:10. In 1975 there were 2,395 entrants, which grew to 7,897 in 1979. There is no way that a blanket qualifying time of 3:00 could be applied and have that many runners in a field, which had women and older runners. In 1975 a German woman, Liane Winter, won with a women's world record time of 2:42.
In 1979 the Boston Marathon had a record of 3,031 runners breaking 3:00, which means that 4,861 finished above three hours. There is no way that a qualifying time of three hours could have been used to screen all entrants. 1980 was the famous Rosie Ruiz race. There were only 5,471 entrants, which could explain a tightening of qualifying times for the elite runners to 2:50. Higdon must have refering only to a certain male class of runners and not the entire field in terms of a qualifying time.