<On my personal scale of things I "discount" all marathoning before the '70s (or even '80s) (see, us old farts don't always think the ancient days were the best!).
Why? Simply because marathons were very obscure bits of competition, contested more by people who were a little strange (and I say that in all kindness) rather than by the incredible athletic specimins we see today. There was just little or no incentive to be a marathoner, so those who excelled didn't really represent the pinnacle of distance talent as they do today.
Bikila was incredible in his day, but his feats pale with those of Geb.>
Hate to disagree with Editor, T&FN. By what thinking does a 2:12 and some marathon run in 1964s most important competition, against all top opponents, pale with say a 2:05 and change race today? Remember also that Bikila's run came about 7 weeks after an appendectomy. When KK, Geb or anyone else for that matter takes down the world best for the marathon by about 3 mins., in the World Championship or the Olympics, I will agree.
Though, Zatopek's 1952 Oly marathon triumph appeared "easy", it is wrong to denigrate this all-time great, compared to the "incredible athletic specimens of today". After Viren, few people have seriously approached a 5/10 double in the Olys and Zatopek went one further. Given his won-loss record for many years, his wins in important meets etc., Emil Z is the track and field athlete of the 20th century in my book.
And the greatest performance is Nurmi's one hour 1500/5000 double at the Paris Olympics. Nowadays athletes get the schedule of races changed to attempt lesser doubles.