Unfortunately, "Once a Runner," John L Parker's legendary book about the plight of a runner, is ultimately forgettable.
It is a search for self and a search for meaning that drives the book, with an existential nihilistic, or, perhaps less philosophically, humanist gear pacing the reading through the selfish individualism of runner Quenton Cassidy. His goal is to both break the 4:00 barrier (a gold standard among milers) and to compete against the world's best. However, he has broken rules which bar him from racing in the key meet.
In general, it is well written. Occasionally has a literary quality, but then drops into average novel-of-the-moment phrasing. It captures the intensity and mindset of the runner fairly well. Drops a lot of the names of known runners (Pre, Liquori, Shorter, etc.)
As it was published in 1978 originally, that could be a problem. As a runner myself, I'm somewhat of a track nerd, and was reading "Runners World" in the late 1970s and early 1980s while those guys names and some of the running sub-culture was as described. Would the world who follows Hall, Cox, Haile (names, which in a few years, may be forgotten by the less-than-hardcore-runner) on the long distance side, or the crowd who watches Webb and so on -- would they 'get it'? The running boom was just kicking into high gear then. Jim Fixx's book, "The Complete Book of Running" had just come out. Coe, Cram, Ovett hadn't really gotten going. Marathons hadn't blown into mega events yet.
As a book, I wonder if non-runners would enjoy it? I'm not sure. I don't think that it transcends the niche audience/market the way Alan Sillitoe's "Loneliness of the Long Distance Runner" has.
Runners who read regularly will devour this, as might the endurance sports enthusiast. Its accolades will remain there. For the rest of the reading public, it is average, if not overdone.