He understood the police were considering charging the driver with careless use of a motor vehicle causing injury.
"He stopped up the road and came running down, just beside himself, freaking out, not knowing what to do. He asked me my name, he kept saying 'Sorry sorry', then he kind of held my hand and he said, 'Stay with us', like I was going to die or something."
There was no cellphone coverage but other drivers stopped and someone phoned for emergency services from up the road. Mr Cornwell said he felt sorry for the driver, who had expressed remorse. He was thought to have been blinded by sun-strike, but Mr Cornwell said drivers in that situation should slow down or stop.
However, Mr Cornwell has only scorn for the Ironman organisation.
"They promote it as being all for the athletes, but it's more of a business trying to make money and the athletes are how they make their money."
The Ironman website says entrants are eligible for a partial refund of $390 for requests received by December 31 and $150 for requests up to January 31.
Mr Cornwell said Ironman should be able to judge cases like his as exceptional. He urged the organisers to consider arranging a withdrawal insurance scheme, which he said was available for some overseas events.
Ironman's New Zealand manager Janette Blyth said that more than one in 10 entrants pulled out "and we cannot place a different weight on the varying circumstances that force a competitor to withdraw".
Ironman Asia Pacific's chief financial officer David Ray said the agreement was clear and many costs were incurred before race day, preventing a full refund or entry transfer.
"We are not doing that to be mean-spirited."
He said withdrawal insurance would be investigated.