I don't particularly like the book (the premise of a "realistic" zombie attack is self-defeating), but I assume the film only shares the name for the sake of brand recognition. Probably a good thing; there isn't that many ways you can really do a zombie story, and most modern movies attempt a "realistic" approach anyway.
There's a lot of people bitching about how the zombies don't move slow, like in the book (or indeed - every fucking zombie movie that hasn't figured out that a monster that moves at a snails pace isn't scary anymore). It is probably for the best that the zombies are not only fast, but move like a god damn avalanche - that's quite a fresh idea, and it even makes kind of sense, if you've ever seen how ants attack something. Why the hell wouldn't they be scrambling over one another? It makes them far more dangerous than any zombie plague ever depicted.