Very good point. The question asked by the disciples was the long sought answer to when messiah will rule as king physically over Israel as promised by the OT prophets and prophecies. That is why the context of the passage as a whole is important - Jesus is answering the disciples' question.
Keep in mind too verse 21 "For then shall be great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be. " How applicable is the destruction of Jerusalem to this? Million of Jews were killed during the holocaust. This alone indicates that the events of AD 70 are not what Jesus was speaking about - but something later.
Just as the OT prophecies of Jesus resulted in real, tangible fulfillments, why would the NT prophecies - particularly those by Jesus Himself - result in anything less tangible?
It is only a problem for dispensationalism to grasp. It is all the same event, which historically lasted for a period of time.