Then they (a)returned to Jerusalem from the mount called Olivet, which is near Jerusalem, a (b)Sabbath day's journey away.
v.12 (b) a Sabbath’s day journey was 2,000 cubits, about 3,000 ft., according to Num 35:5. The Mount of Olives was within this travel limit, Acts 1:12, but Luke 24:50 states that He led them to Bethany and was ascended into heaven; according to John 11:18, Bethany was about 9,000 ft. from Jerusalem. So we have a discrepancy of 6,000 ft. Any time you come across something that seems like a contradiction in the Bible, use it as a chance to dig deeper because the Word of God does not contradict itself.
John 11:18 is focusing on Jesus raising Lazarus from the dead. Mary, Martha and Lazarus lived in Bethany and this was a common place for Jesus to stay when He was in Jerusalem. The town of Bethany was not big, but was at least bigger than 2 miles. I believe that Mary and Martha’s house was in Bethany, two miles from Jerusalem, while the Mount of Olives and the place of the ascension and Bethpage, were within a Sabbaths day journey from Jerusalem, but still within the borders of the town of Bethany. So when the Bible states that Bethany is two miles from Jerusalem, Jn 1:18, and also states that the disciples were a Sabbaths day journey away from Jerusalem, but still in Bethany, these statements agree because the town of Bethany was bigger than two miles.