Is it an obstacle course type race, where you have your zombies, and you have your survivors? And the zombies try to catch the survivors and "infect" them? If so, an hour isn't that unrealistic - my first 5k was an obstacle mud run, and despite the fact that I can usually do a 12-13 minute mile and was in pretty good shape, it still took me 49 minutes (it was the hills that killed me!). Many other people took 1.5 or even 2 hours. When you sign up for obstacle runs, they generally advise that if you think you may take longer, choose a starting wave earlier in the day (at the very least not the last 2) to give you plenty of time.
If it's more of a flat 5k and everyone dresses up like zombies, I don't have experience there but I'm sure you'll have your runners and your shamblers, and everything in between. After all, not all zombies can be fast - that would be blasphemous.
If it's an event that happens year after year, see if you can find previous photos/finish times/articles on it - anything that will give you an idea of the feeling of the race (if it's super competitive, just a place for people to dress like zombies, or everything in between) - though fun runs or ones that require/encourage costumes generally aren't super competitive.
Do something everyday that your future self will thank you for.
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