HIHS. You don't suddenly switch from burning glycogen to burning fat. You're burning a percentage of each all the time. You can't consume enough carbohydrate to offset the amount you use anyway. You need around 3000 calories but you can consume about 300 an hour from Gels.
The theory is that after two and a half hours you've gone through all your glycogen and your body has been running on fat for an hour. No further adaptations are going to take place. All that happens is you get tired and risk injury. The 20miles figure is a hangover from a time when most marathon runners were running much faster times.
How's the calf
A 3:30 marathon runner will do 20miles in 3 hours.