Economic hard times
Some people worry about the fate of races during economic downturns. When folks experience financial hardship, they tend to be more self-focused and cut back on contributions, memberships, subscriptions, luxuries (if they have any), and anything else that isn't absolutely necessary.
Runners will always run, but they may race less often. They'll still buy running shoes but not other gear like shirts, shorts, skin balm, watches, or heart rate monitors. They'll make do with what they have.
If a runner registers for only one race per year, make that one race be your race.
Offer something unique that can't be obtained at another race - e.g. great food, cool t-shirt, live entertainment, exceptionally scenic course, attractive distance, great premium, or valuable raffles.
Be prepared for more bandits (runners who compete in the race without paying an entry fee). Keep track of bandits who cross the finish line and make sure they are not listed in the results or receive an award - even if they would have qualified for one. Even better, don't let them through the finish line chutes.
Runners won't stop racing. They'll just be a lot more choosy about which races they enter.