It's probably been thirty years since I read the Richard Bachman (wink wink) short story, "The Long Walk." If you asked me which Stephen King stories need be adapted to film, this would have been low on the list. I mean, it's not a very cinematic concept. The majority of the story is just watching people walk.
I don't remember the book enough to tell you how close they got it. But I do remember one thing: The book was very bleak, and made me feel physically tired just reading it. And in that respect, the movie captures the spirit of the book quite well.
Since the story was so simple, I kept expecting them to stretch it out with Grinch-levels of flashbacks and other superfluous content. But they didn't go there. There are a few short flashbacks, but the majority of the movie is centered on the dialogue between the walkers. In a strange way it reminded me of Stand By Me.
Visually it looks like an episode of The Walking Dead, with similar costumes and color filters. The gore is explicit and sickening. I was about to say it was unnecessarily gory, but the frankness of it does help you feel like you're walking in the characters' shoes, so to speak. You feel their horror.
Is it a good adaptation? Yes, it's damn near perfect. Will I ever watch it again? Absolutely not. This is a fantastic movie, but it's hard to call it entertainment.