It has to move from an adjacent tile to an adjacent tile.
I just had an RnR game where I set up a Maginot Line. I had some mountain tiles that I captured that weren't roaded and therefore my wheeled units couldn't use them. So I set up a 'Valley of Death' (It wasn't a valley, but mountain ranges) where I had several mountain tiles - I think three deep - and force the AI to run a gauntlet through them, where my tanks were fortified on the non-mountain tiles around it. This did help forcing them to take potshots at them, and I had arty waiting for them outside of the mountain ranges. A lone worker was the bait, and I ended up never having to move him because of the combination of arty/bombers/tanks. It was sweet.
Didn't last long tho, because I ended up taking down a lot of the enemy AI and swept through their territory like wildfire.