“Simply put, I broke mentally,” Riddle said.
“I was composed. I thought I was prepared for what was to come.
"I believed I was ready.”
Riddle entered the race with confidence after what he described as the most preparation he had ever put into an event, but the demanding conditions exposed areas of his racing that still needed work.
The biggest physical issue came on the bike when a longstanding sciatic nerve problem returned.
Riddle said there was no pain, but he lost feeling in one foot and struggled to produce power.
“Early on into the race, I was so close to pulling out because I just could not push any power,” he said.
He eventually managed to settle the problem and worked his way back towards the front, but the effort added to tactical decisions that proved costly.
A three-minute penalty followed a lapse in judgment after he pushed hard to close a gap to a group.
From there, he found himself repeatedly forcing the race, eventually reaching a point where his body could no longer respond.
“I just go almost all out to try and catch some group, making stupid decision after stupid decision after stupid decision, until I can't any more,” he said.
By about 110km, Riddle said, his power had fallen dramatically and he could no longer hold his riding position.
For roughly a further 50km, he wrestled with whether to continue before deciding the risk to his career was too great.
“It was truly the hardest decision I've ever had to make because I have been dreaming of this for so many years,” Riddle said.
“I pulled the plug and I regret it with everything inside of me right now."
He also acknowledged that his expectations had become part of the problem during a season that failed to match the standards he had set for himself.
“This whole year I've put too much pressure on myself,” he said.
“I'm still a student of the game and to think anything else is a bit too confident.”