If the world were truly ending, fleeing to the mountains would be pointless. You can't escape global destruction by running to higher ground. But Jesus' words in Luke 21:20–21 are specific and practical:

"But when you see Jerusalem surrounded by armies, then recognize that her desolation is near. Then those who are in Judea must flee to the mountains..."

This is a geographically targeted warning. He's not talking to the whole world, He's warning those in Judea about the coming destruction of Jerusalem. And history confirms that this happened in 70 AD, when the Roman armies surrounded and destroyed the city.

If Jesus were describing the end of the planet, instructions like "flee to the mountains" would make no sense. But if He's describing the judgment on Jerusalem, it makes perfect sense, and it was fulfilled just as He said.