I saw the program too, but if one is fasting and you go out for a ride say 20 miles + would you not bonk as you will be running on empty
Quite possibly.
Extreme dieting/fasting does not go hand in hand with aerobic exercise.
You may be OK doing 20 miles in the morning if your previous day was a normal eating day (I don't have a problem cycling before breakfast) but I think you'd run into problems doing it later in the "fast" day. Also 20 miles would burn 300-500 calories (depending on how hard you went at it) so that's going to make a significant dent in your blood sugars. You may feel very run down later on.
Certainly doable in the morning but I bet you'd get close to bonking if you did it in the afternoon.
I'm currently dieting and had to dive into a Tesco Metro last night, after an hour on the bike, to cram a sandwich and chocolate milk down. I was feeling a bit shaky. Amazingly, after Burger and Chips + 4 pints of Bitter, the ride home felt much better (Note. Not part of my normal diet)