I wish I knew why Jesus and the prophets don’t seem to distinguish between near-future and far-future foretellings. Back and forth, Jesus talks about the destruction of Jerusalem, an event some 40 years ahead of His life and death, and He also speaks of His return upon the clouds, which as yet still hasn’t happened even after some 2,000 years. Isaiah did the same thing in his Messianic prophecies, telling of Jesus’s First and Second Comings in the same chapter, sometimes even in the same verse. Since I can only guess, I will guess it’s similar to the reason for speaking in parables, so that the ones given understanding by the Holy Spirit will see and understand, while everyone else will just be confused.
After encouraging His disciples that they will go through persecutions but will not perish even if they die, Jesus gives warnings to those who will live through the sack of Jerusalem. Anyone inside the city should flee, and those who are outside should go to the mountains for safety. Many will be killed and taken as captives and the city will be trampled underfoot by the Gentile armies.
Immediately after explaining these things, Jesus talks about the signs and portents of the coming of the Son of Man (i.e., Jesus Himself). The great changes in the cosmos will cause everyone on earth to fear, fainting and trembling, because they have never seen anything like it. But then Jesus will return in power and glory, and His chosen people are to raise their heads because their redemption is coming along with Him.
Come quickly, Lord Jesus. Rule and reign in righteousness and fulfill all of Your promises.