It generally takes a couple of weeks for your body to adjust to keto. Also, you may have been pushing it a bit harder than you're used to.

I'm not a "keto purist" since I do LCHF because I'm pre-diabetic, not for losing weight (although I have lost 42 pounds in the last 18 months). I generally do around 30 to 35 carbs a day. I found that I can do a 4 hour ride without problems if I take it really easy. If I'm pushing it, I have to have a boost about 2 hours in. I use the Kirkland dark chocolate nut bars, which have < 10 net carbs. Also, you really have to pay attention to your electrolytes and fluid intake. Keto may cause you to dehydrate a bit, as well as run short on electrolytes, so you have to compensate. I use Hammer's Endurolyte tablets, because they are zero carb.

You say you had a zero carb breakfast and zero carb dinner. I get virtually 100% of my carbs from vegetables. Some people on keto overload on protein, and don't realize satiety has to come from fat in the diet, not protein. Veggies are important as well -- vitamins, minerals, etc.

I can't "carb up" before a ride, because that would cause a spike in my blood glucose. I can have a protein bar in the middle of a hard ride because the extra glucose is used up fairly quickly.

BTW, Chris Froome was on LCHF when he won the Tour, though I'm not sure if it was Keto or not (<20 carbs/day).