IMHO that's a short enough ride that you probably just started to get tired. I can ride pretty well for about 2 hours on no food at all, but then I'm toast. So you under ate a bit, no big deal. It will become a big deal as your mileage increases.
I believe the 250 cal/hr thing is for average riders, so maybe you should eat more. The rule is actually about how fast you can empty your stomach. Most folks can't get rid of more than 250 cal/hr. If one eats too much, one bloats up, the stomach stops emptying, and one starts to bonk from too much food rather than too little. I'd guess you're so big that you can probably do more food than that. You'll just have to experiment. Every one is different - thankfully.
I don't count the "before" food, because I try to eat my last food 2-3 hours before riding. Eating right before can cause a blood sugar crash. If you do like I do, your blood sugar levels will tend to stay more even.