Keto is more an eating protocol than a diet. Your body switches to running on fat, rather than carbohydrates. I know a guy who did a few brevets powered solely by olive oil. He had an excellent understanding of what he was doing though, and regularly tested for ketone bodies in his urine to verify ketosis; it's not something you should do without understanding what is going on in your body.
I am not in a keto eating protocol, but I am in a a 16:8 to 20:4 intermittent fasting eating protocol (16 to 20 hours of fasting with a 8 to 4 hour eating window). It varies, though.
Still, I ride 30 km commutes (week days) and 100 km metric centuries (weekends) in a fasted state all the time. I am on flat paved bike paths 90% of the time and in a comfortable zone 2. Once you are adjusted you're fine, but if you go straight from regular 3 meals a day eating, with unlimited carbs, to time restricted eating, you can get a little light headed after a decent effort on the bike. That's low blood sugar, and if you can get through it, your body starts to use fat.
Once you are into a good routine, such as eating from 12.00pm until 8.00pm with predictable efforts, like daily commutes, your body adapts to burning fat if it depletes its glycogen stores and you don't bonk as hard or get all lihht headed.
It's not for everyone, but I am coming up on 50lbs lost with the best health check up numbers I have seen in years. Those blood tests don't lie.