I weighed 360 pounds in my best days and I am a scientist, and I say, with authority; "Nonsense!, on several levels." Nobody could survive on 20,000 calories for any amount of time. Why not? Well, the unit you are thinking about actually is kilocalories, and he would eat 20kcal when the recommendations are somewhere between 2100 and 2700kcal per day, more if he really runs like a maniac. And 20,000kcal is just as impossible because there is no way he could metabolize that much during one day unless he is on fire.