The climax of the story occurs when the protagonist believes she is being chased by real zombies and starts running to save her own life.
We can arrive at this answer because:
The climax of a story refers to the tensest moment in the story.
This moment comes when the main characters are facing their greatest challenges.
In "Running for Real (Zombie 5K)" the climax occurs when the protagonist believes she is being chased by real zombies and needs to run to save herself.
This is the climax moment because the protagonist spends much of the story pretending that she is running away from zombies, until the moment when she really needs to do it.
