Let me try to offer my simple understanding, presented in a simplified way. Someone can quibble that I've neglected some details if they like.
You have a limited supply of glycogen (stored broken down carbohydrate) in your leg muscles. Glycogen is one source of fuel for running at marathon pace. The other primary source of fuel is fat.
Glycogen is a much more efficient fuel for delivering energy to the legs than fat at faster paces. When you run out of glycogen, you have only fat left to burn. At this point, you can no longer maintain faster running paces (like M-pace), and you "hit the wall."
The ratio at which you burn glycogen:fat depends on your pace. The faster you go, the higher the glycogen:fat ratio. Therefore the time/distance you can run is limited by the pace you are running. If you run faster than your actual marathon fitness, you will run out of fuel (glycogen) early and hit the wall. If you run slower than actual fitness, you won't run out of fuel before the finish line and will probably have managed negative splits. If you nail your optimum pace for the whole race, you are theoretically hitting the wall just as you cross the finish line.
The rate at which you burn glycogen:fat can be adjusted/improved through training. The amount of gas you have in the tank (glycogen stored in the legs) can be increased a little bit in a couple of ways: not wasting energy with a vigourous warmup before the marathon; taking in fuel during the race; and, optimal fuelling prior to the race.
Some people believe that the depletion/reloading technique allows a temporary increase in glycogen storage, so that you can arrive at the line with more fuel than you normally have on a day to day basis. I don't understand the physiology behind this effect, but personally I believe it to be true.
Anyhow, that's what I know. Or think I know.