What does it mean to "flee to the mountains" as writen in Mark? Mark 13:14
  New International Version (NIV)
  14 “When you see ‘the abomination that causes desolation’ standing where it does not belong—let the reader understand—then let those who are in Judea flee to the mountains."


First off, is this meant figuratively or literally? I lean towards believing it is literal. But then it specifically mentions that it is those in Judea that should flee... is this also literal Judea? What about those of us in the nations, now?

I do not like Jehovah's Witnesses' explanation that they are the symbolic mountains and that by joining them we are already 'safe'; I believe there is more to it than that. For instance it is only when we see the abomination causing desolation that we are meant to flee, so that does not make sense if you compare it to joining a religious group.

And what of people living far from any mountains? This is where it is difficult to believe that it is literal, yet the detail of the rest of the verses in Mark do suggest that actual fleeing is necessary.

I'm a little stumped. What does "fleeing to the mountains" actually mean in this context?
You get the sense of literal fulfillment by reading the verses before and after. Jesus is not switching back and forth between the two in the chapter when He talks about what will happen in the last days. When Jesus told His disciples that the temple will be torn down (13:1-2), the disciples wanted to know when that would happen, and what the sign will be for it's fulfillment (13:4). In other words, Jesus was talking about a real temple (the one they were looking at, that Herod built), and real, discoverable signs concerning its end.

The events in the verses immediately after that are easily discoverable in our literal world and relationships. In verse 14, Jesus starts answering the second part of the disciple's question about signs of the fulfillment. That sign refers to an event mentioned in Daniel 9:27, from which Jesus gets the phrase, "abomination that causes desolation." Some pretribulation groups claim that this event will take place when an antichrist figure will set himself up as a god to be worshiped (taking their text as 2 Thess. 2:3-4). When the Jews see that literal sign, they are to flee out of Judea, which is surrounded by hilly, mountainous terrain. Read again verses 15-18; Jesus indicates in these verses that the fleeing is literal, and extremely urgent. If you are on your housetop, don't stop to pick up stuff on your way out (v15), if you are in the field, don't stop to pick up your garment (v16); if you are pregnant or care for little ones, it will be hard for you (v17); and "pray that your flight may not be in winter" (v18). All of these details emphasize the literalness of the event in question, of the urgency of fleeing when the sign is seen. 

The literalism comes to an end in the chapter when Jesus deliberately injects a parable, and says He does so ("Now learn this parable from the fig tree"). Once the brief parable is over, He goes back to using terms that can be understood literally. 

In some cases of prophecy, they have a near fulfillment and far fulfillment. It is so with this prophecy. In its near fulfillment, the temple was indeed torn down as He said it was, in 70 A.D. by the Roman armies of Titus, and those Jews who fled when they saw the armies surrounding the city (a detail recorded in Luke's version of this chapter of Mark, Luke 21:20-21) escaped to safety, and were spared the grievous horrors suffered by the Jews left behind recorded by Josephus. 

However, the Abomination of Desolation was not set up at that time. That will be left for a future day, which must take place after another literal temple is built in Jerusalem. After that event, according to the sequence of events in Mark 13:24-27, Jesus will come again.