The second Wall is in the middle, the Wall of Sleep. When we reach this Wall, we either break through or crash into it. If we run at it at our full speed, we're most likely to crash and burn by falling into a deep sleep of exhaustion. The effects of hitting this Wall occasionally aren't permanent: everyone reaches it at some point, and people often push through it without really being able to articulate what they have just done. Hitting the Wall constantly, though, is generally unhealthy. As you pound yourself against it over and over again, you find yourself never quite backing away, and when you do, you realize that you've simply given your approach more time to accellerate. People who have seen the other side, where vision is clear, choices seem easy, and reality is remarkably uncomplicated, often seek to get to the other side of the Wall. It's an attractive place, but humans aren't meant to live there. The longer you are there, the closer you get to the third Wall. Mystics and prophets spend much of their lives beyond this wall.
The third wall is at the end. This is the wall that no one is meant to touch. This wall is the Wall of Death, and to be crushed against it brings hopelessness, fear, loss, and anger. One can reach it without dying, but one cannot pass it and live. Those who have gone beyond and returned are not living the same life they were. Think Christ, whose ressurection transcended this wall, or Mohammed, who's ascention into heaven is not a death, but a movement beyond life. Those who go beyond the Wall of Death are no longer recognizable as human, but are seen as divine.
The time you spend between the Walls of Sleep and Death is inverse to the distance between them. The longer you are beyond Sleep, the closer Death looms. Spending time between Birth and Sleep will pull Death back, but at a slower rate than you draw it to you. I'm tempted to believe that the longer you spend close to Birth, the further the Walls of Sleep and Death get, but I imgaine that all these Walls march closer, with only Birth remaining forever where it began.