Then what motivated the "First Runner", Greg? Where was he/she running TO?
The idea of Sanctuary as a delusion on the part of the Runners and/or the Computer makes no sense.
Why does it make no sense?
Some guy decides he doesn't want to give himself to Carousel, so he tells a friend he's going to escape.
Guy attempts to escape, and since he's gone, the friend assumes he escaped (Not knowing he was caught and frozen).
Friend tells some people, who tell some peope and a myth is born, which grows with every retelling (Ever try an experiment about how different a story is from when you tell it to someone, and it goes around the room, until it comes back to, wildly exaggerated?)
Computer, not being able to think but only follow it's logical programming, now knows there is this legend, logic says it must be true if so many people believe.
Computer is now perpetuating the myth, by freezing the folks trying to escape, because the populace believes those escapees have actually escaped.