Jesus was saying that the preaching of the gospel throughout the world was a sign of that the destruction of Jerusalem was near. God's purpose in giving the sign was that all the nations would understand the significance of the fall of Jerusalem (cf Exodus 32:12, Deut 2:25).  

There is some confusion about the word "end" in this verse.  The Greek word translated as end is "telos", which means end , last part, close or conclusion.  It is not the same as the world "sunteleia" which is used in the expression "end of the age" in Matthew 24:3.  That word, although often translated as "end", really means consummation.  The disciples were asking about the end of the age so it was the appropriate word.  In verse 14, Jesus is not speaking about the end of the age, the consummation, so he just uses the word telos.  He was warning of the end of the Jewish nation in its current form, so this word was appropriate. 

Jesus gave two signs of the destruction of Jerusalem. The first was the preaching of the gospel to the nations. The second immediate sign was Jerusalem being surrounded by armies.  The people of Jerusalem should escape to the mountains (Matt 24:15,16, Luke 22:20,21).  Both these signs were fulfilled before the destruction of Jerusalem in A.D. 70.  Jesus warning saved the lives of many believers.  Although a million Jews lost their livens in the siege of Jerusalem, none of them were Christians.  They heeded Jesus' warnings escaped with their lives.

Some people may find it hard to believe that gospel had been preached to the whole world by A.D.70 when Jerusalem was destroyed.  Yet this is exactly what the Bible teaches.  On the day of Pentecost there were devout men in Jerusalem "from every nation under heaven" (Acts 2:5).  They would have taken the gospel back to their own lands.  In Romans 1:8, Paul gives thanks the Christian faith is spoken of "all over the world".  Colossians 1:6 says that "all over the world the gospel is producing fruit.  The most specific fulfilment is given in Colossians 1:23.

This is the gospel that you heard and that has been proclaimed to every nation under heaven...

This sign of the preaching the gospel must not be confused with the Great Commission, which required disciples to be made of all nations.  That task is still not complete, but Jesus  prophetic sign only required the gospel to be proclaimed as a "testimony" and a witness before the destruction of Jerusalem in AD 70.    An examination of the Bible shows that Jesus words were fulfilled. 

It follows from these points, that Jesus was not saying that the preaching to all the nations is a sign of his second coming.  Those who use this verse to support this view are interpreting it incorrectly.  There is no scripture that states that the second coming will be proceed by a great end-time harvest.

God has commissioned us to make disciples of all nations and establish his kingdom on earth.   Christians in all centuries have a responsibility to work on that calling, which is far from complete.  Even if the kingdom of God is established in our lifetimes, there is no reason why the second coming should come straight away.

We are seeing a great advance of the gospel in our time.  A great harvest is taking place.  We should rejoice in this,  but it is a mistake to see it as a sign of the second coming.  I believe that we are getting close to a great victory for the kingdom of God.  We will see an even greater harvest in the days ahead, before God's purpose is complete.  God's purpose is not just to harvest the earth.  The harvest is not an end in itself.  They farmer completes the harvest, for what comes afterwards.  God's purpose is that the whole earth will filled with his glory, as his people establish his kingdom throughout the earth.