Some good advice here.
I think my own objection to the language "I failed" is that it doesn't contain the lesson. It's not that there's anything wrong with failure. But there's something wrong with the attitude that failure is or was inevitable. To me, connotationally, there's a world of difference between "I failed" and "I made a mistake" (or two mistakes).
Training for the heat may not have been possible - I'm not sure what the weather has been in your area, but around here the temperature has been really variable this year. Right now it's about 55 degrees outside. We've had some hot days, but never three in a row yet this year, so none of us, even the ones who ride 40+ hard miles daily and at least one century every weekend (which I'm not one of those! :rolleyes:), is really acclimated to heat yet.
Training for the distance was possible. Acknowledging that you were undertrained and selecting a shorter distance for the event was also possible. To me, that wasn't a "failure," it was a "mistake," and it's important for you to identify that to yourself. "Failure" focuses on the past, self-blame and disappointment. "Mistake" focuses on planning for the future, and taking steps to be sure that if you fail again, it won't be for the same reasons.

