tricialit
You essentially walk the parts of the route that will-
A. Sap your energy most if you run them
B. Give you a minimal return in terms of speed over walking them.
For example if you run up a very steep hill, you will be slowed down to little more than walking speed, and your legs will be totally shot at the top of it. It makes much better tactical sense to walk up it, suffer a slight penalty in terms of speed, but have a recharged set of legs at the top.
In a hilly event, I walk up the hills but run the flats and downhills. In a flat event I will walk sections that are very boggy, or where I am going into a strong headwind. In a longer event with a flat profile, I might have to adopt a strategy of run 25 minutes then walk 5. As I get weaker, the ratio might shift to 20/5 and then 15/5. I don't expect this to be necessary with a 33 mile event though. Just walk anything where the game wont be wort the candle and you will be fine.