Donner is only 1 hr long…but really only 36 minutes of “intensity”…or roughly 600 Calories.
Your porridge would have provided perhaps 300 Cal plus your muscles would have provided at least another 1,000+ Cal so you should have had more than enough carbs onboard to fuel the workout. I’m guessing it was something else besides low glycogen stores which caused your “bonk”.
Properly fed, I almost never have to feed during a trainer workout.
Here’s a free Time-in-Zone/TSS calculator that shows Calories burned for the workout (which TR also does); you can use that info to fuel properly pre-workout. 