Its the shortest route from (the relevant number) to the bottom-right that doesn't go through any of the guardian balloons ... if you take the grid as a maze with thick 'walls' that you can't go through.
Very clever puzzle. Also a slow start but spotted the location and thus significance in good time and so the bottom line then helped finish things off. It also suggested what the quote was going to be.
Well, it would have done if I hadn't sloppily put an 'm' instead of an 'n' in 3d. That rather hampered the escape (which I worked out in reverse).
Biggus, my shortest route from the number was 3 cells down, 4 right, 2 down, 1 right, 3 down, 1 left, 3 down, then 1 right.
Someone please point out the error of my ways.
Not sure if this is the correct solution to the 'route' but the first 5 letters of the quote lie on a diagonal running down and left of the number, each letter being reached using down/left or left/down moves. Then down two, right two, down two and then diagonally again to 40 ac. to finish. This is clearly not the shortest route through the grid though!
Nelson, Biggus - you need to think of the grid as a naze - you cant go through the thick 'walls'... and nor can you go through a guardian balloon. It is the shortest route given those restrictions (and a very cleverly constructed puzzle!)