I think they will manage to defeat the zombies in the end but Michael gets killed in the process. We flash forward 20 years and the world is starting to be rebuilt and the person reading the journals is Michael and Pegs son/daughter. They have an emotional conversation about about their dad as someone goes running bare ass down the street while being chased by a zombie!

Pegs says "aw shit, not again", takes out her shotgun with "Michael" engraved on the side of the barrel, goes outside and blows it away

That's probably what's going to happen right?