The destruction of the temple, which is what Jesus is talking about, is not a future thing it happened back then.
Actually it can be split into two things, prophecying for the short term to those who are listening and then a more general prophecy for us today. We need to divide God's Word rightly and sort out which parts are for them living in Jerusalem, and which are for us, living in America, or Australia, 1900+ years later.
It is clear that Jesus is speaking to the then Jews, as he refers to the Sabbath which the Jews kept and it was forbidden to travel long distances on the sabbath:
Mat 24:20 But pray that your flight is not in the winter, nor on the sabbath day;
Jesus predicts the coming invasion of the Roman armies right here:
Luk 21:20 And when you see Jerusalem compassed with armies, then know that its destruction has come.
Now what happened historically?
In the first Jew-Roman war, Cestius Gallus besieged Jerusalem suddenly, then withdrew his army very much unexpectedly, which provided the Christians an opportunity to escape, which they did, and many, remembering the words of Christ, fled to mount Libanus and so were saved. Such that there was not a Christian left in the city, and so were spared. In answer to their prayers (as Jesus told them to pray it would not occur in the winter), it occured in the summer time.
The many caves in the mountains of Palestine provided a safe refuge for those who fled.
The theologian Barnes says of this:
The coming of the Son of man - It has been doubted whether this refers to the destruction of Jerusalem, or to the coming at the day of judgment. For the solution of this doubt let it be remarked:
1. that those two events are the principal scenes in which our Lord said he would come, either in person or in judgment.
2. that the destruction of Jerusalem is described as his coming, his act.
3. that these events - the judgment of Jerusalem and the final judgment in many respects greatly resemble each other.
4. that they "will bear," therefore, to be described in the same language; and,
5. therefore, that the same words often include both events, as properly described by them.
The words had, doubtless, a primary reference to the destruction of Jerusalem, but they had, at the same time, such an amplitude of meaning as also to express his coming to judgment. See the introduction to Isaiah, section 7, (3).