We saw that the "abomination of desolation" that Jesus predicted is one of the main hinges as to why the Gospels were written post 70 AD, but if they were post the event, you would think that the authors would have Jesus giving clearer and a more accurate advice. Matthew 24:15-18 and Mark 13:14-16 has Jesus telling the people in Judea to flee to the mountains when they see this "abomination of desolation." The only problem was, that by the end of 67 AD, we learn from Josephus's The Jewish War (written around 75-80 AD) that the Romans controlled the hills.
Flavius Vespasian had an army and in 68 AD and was waiting for orders from Nero to take out Jerusalem, but Nero committed suicide (9 June 68). Galba took his place but was killed (15 Jan 69) by Otho who then ruled but also committed suicide (16 April 69). Vitellius then became the Emperor until Vespasian got jack of it all (until June 69). Vespasian, who was twiddling his thumbs (not really), still waiting for orders, named himself Emperor and left his son Titus to take out Jerusalem. (Josephus was tight with Titus and so was a first hand eye witness to all that was going on in Jerusalem).
If Jesus's advice was to flee to the hills when people saw the army coming, it would mean that they would have been told to run towards the army occupied territory. Pretty bad advice really. In fact the Christians did take some advice about fleeing from Jerusalem, but it wasn't to run to the hills as Jesus said. The advice they received (and took) was given "before the war" (making it before 66 AD) to run to Pella, according to Eusebius's Church History (Book III Chapter 5.3) (some people say he was quoting from the Memoirs of Hegesippus that was written around 165-175 AD). For some more info on the Christian movement to Pella see here and here.
In AD 70 everyone would have known that the hills were in the hands of the Roman army at that time. An admonition to run into the hands of the Roman soldiers about to attack Jerusalem would make no sense. (p94)Overman then quotes Robinson:
"It is clear at least that “the abomination of desolation” cannot itself refer to the destruction of the sanctuary in August 70 or to its desecration by Titus’ soldiers in sacrificing to their standards. By that time it was far too late for anyone in Judea to take to the hills, which had been in enemy hands since the end of 67." (p94)Don't like this reason for the pre-70 AD date of the Gospels? There are more...