Running under and over 20 miles has everything to do with running on glucose. The capacity of your muscles to use fat depends on the number of mitochondria. The number of mitochondria can grow when you exercise in ketosis.

People that have always done a high carb diet don't have enough mitochondria (glucose burning doesn't require them). So when you have depleted your glycogen stores after 20 miles, you crash.

But when you have invested in a bigger capacity to burn fat (by increasing your mitochondria) you get all your energy needs - even when running - from fat. And human potential for creating fat reserves are - as we can see around us - unlimited.