Matthew 10:23 reads: But when they persecute you in this city, flee ye into another: for verily I say unto you, Ye shall not have gone over the cities of Israel, till the Son of man be come.

Jesus did not here promise he would return to earth before the death of the Apostles. Luke 10:1 clarify what he meant:

Luke 10:1 After these things the LORD appointed other seventy also, and sent them two and two before his face into every city and place, whither he himself would come.

In their first mission, they would not have gone through all the cities of Israel before he would come to those cities, i.e they were not alone he himself would later evangelize those places.