Marathons are first-person phenomena, hard to write well about. But there are exceptions, good online journals that occasionally feature their authors' distance running experiences. Some at the top of my current reading list, full of humor plus fine prose:

TopicRunning - TopicJournalizing - TopicWriting - 2004-02-08

Inactive (or less-active, currently):

(correlates: PsychologicalUndead, ClassicCorrigendum, TommyBurgerRevenge, ...)