I've tried biking a few times before breakfast and didn't like it even on a 30 or 40 minute ride. It wasn't energy that was lacking but the feeling of hunger became distracting.

I found that making 5 or 6 strips of bacon before a ride takes care of that. I also found the same amount of food will hold me for a 2-1/2 to 3 hour ride as well!

It's one of the great advantages of being keto-adapted that you don't need a constant infusion of fuel -- you already have it. If the fasting state cardio works for you, that's great!