For nutrition, the general trend to follow is, the harder and longer you are hiking, the more simple carbs you need to be eating. This goes for any endurance activity.
The higher the intensity, the larger % you are burning energy from your muscle glycogen stores. When these run out, you will hit the "wall" as well known in marathon runs for example. You simply have to slow down when burning energy from fat.
So the key is to stop those glycogen reserves from running out. How do we do this? Carbs. Simple carbs. Sugar. Bread. These substances (foods with high Glycemic Index ratings) are the best for the same reason they are looked down upon when eating while sedentary: They will be absorbed into the blood stream very quickly.
Carbs with fiber take longer to breakdown (and may cause stomach issues), and fat and protein have to go through other chemical reactions before getting into form for usable energy.
Obviously, I'm talking about extreme case here. But since this thread was based on the idea of a "death march", trying such a hike would certainly need to follow this mantra. I think that's where AlanK's classic "thank god for GU" came from while doing some very strenuous hike. GU is all simple sugar. Cyclists, marathoners, etc... consume this sort of thing during races. Not fat and protein, or even carbs with a lot of fiber.
When I learned how to apply this, I was able to work out very intensely 4 days a week, with my hikes considered a "moderate" workout. 2 rest days, and I don't fatigue, because I make sure I recover properly by eating carbs after working out.
That said, I don't really do this on hikes. I mix a sandwich, jerky, cliff bar, trail mix, etc... (things I like!). I can go 6000 ft without eating and not be affected.
But when I did cactus to clouds (~11000 ft gain), I ate a decent amount during the hike, but not a lot of carbs, and I started 'hitting the wall' the last 1000 ft. In that case, it's good to have the knowledge that to be successful with those extreme hikes, carry a few French baguettes with you!