I cycle 4 days a week for 2 - 4 hours each ride. I have tried various kinds of fasting including 16:8 and 5:2 and they've all been a disaster. I have extensive split times for the same rides and after accounting for wind/temperature, when fasted, I slow down about 3% after 90 minutes. After 2 hours, I slow down 5%. After 3 hours, I slow down 15% and feel terrible. I also feel more tired for the rest of the day. Even if I don't fast on ride days and fast on the other 3 my ride times are slower and my recovery is harder. I've kept up the fasting for at least 3 months to see if my body will adjust, but it does not work for me. Now, I practice 16:8 in the winter when I'm riding less but not during the main season.

Exactly. Your body is very good at storing energy (fat). It's also pretty good at burning fat while running, especially at the slower pace that is typical of long distance running. You can eat a couple of thousand kcal in one sitting, but you can store a hell of a lot more around your waist. Getting enough water and electrolytes during a long distance run is much more important than having breakfast beforehand.

It's "obvious" to those unfamiliar with fasting. Once you're used to fasting, what was previously obvious and intuitive gets thrown out the window.

Well, it was obvious to me when I started dry-heaving after going on a morning run without eating first. It was a distance/pace that I never found particularly challenging under normal conditions. It's interesting that other people seem to have no problem exercising on an empty stomach.

If you're not used to it it might feel weird. But you absolutely can do intense exercise without eating first.

Does your dry heaving mean anything other than you were feeling stress? Or that your abdominal neurons were disoriented?

I imagine that if you're training for an endurance-based competition, getting all of your required calories in one sitting once a day will either not happen reliably, or be really uncomfortable.

There's a lot of calorie in fats and sugars. It's quite easy to eat a single meal at some place like Carls Jr or Burger King and meet your calorie needs for the day of a run...and then some. If you eat at Cheesecake Factory, it's hard to find a meal which wouldn't satisfy your calorie needs for an several days.

Endurance training involves eating a lot in any case. It's not that difficult. Fasting for 16 hours would still give you an 8 hour window to down the foods.