The Temple and the End
I think the best way to think about Jesus’s words about the destruction of the Temple is to understand him talking specifically about the Temple’s last days while at the same time talking about the Last Days themselves. He is going back and forth between the two: he is telling them what to expect when the Temple is destroyed, and telling all of us what to expect then The End begins.
The Temple was destroyed by the Romans in AD 70.
The End? No one knows the day or the hour, so be ready.