You didn't bonk; you're just not in shape for that type of activity. The cramping was also likely due to that rather than hydration. Keep plugging away and you'll be able to handle it better in the future.
A true bonk is really unlike anything else. I mean, it can be bad enough where there's not enough energy to get off the bike sometimes. And then, 10 minutes after eating something, there is. It's pretty wild.