Paula Radcliffe would only say she was "devastated" as she left the Panathinaiko stadium in Athens after pulling out of the Olympic marathon.
The pre-race favourite succumbed to the searing heat during Sunday's race and was put on a saline drip.
A spokesman for the British Olympic Association described her as "emotionally and physically drained".
Fellow Briton Liz Yelling said she had been surprised by the ferocity of the 26.2-mile marathon course after finishing 25th.
"It was awful," Yelling said.
"I went over the profile and there was supposed to be some flat spots - I didn't see any.
"I am gutted for Paula - I know how hard she has trained, how much dedication and sacrifice she has made. She wanted this one so badly."
David Bedford, the former British 10,000m world record-holder, is convinced that Radcliffe's task was impossible.
"I think it is well nigh impossible for a northern European to win in these conditions," he told The Times.
"Up to 10,000m it could possibly be done but by the half-marathon distance, it becomes too demanding."
Bedford, now race director for the London Marathon, added: "I think the gap in Paula's superiority was not enough to counteract the heat problems that she encountered.
"When northern Europeans run in this intense heat, they get a tremendously dry throat, which any amount of drinking will not affect.
"In the later stages, competitors will get a coldness on the outside of the body and dizziness in the head.
"The legs will increasingly begin to feel weak and they will have trouble running on a straight line.
"But Paula took it on, gave it her best and this does not mean that she is not a great athlete."
UK Athletics endurance coach Alan Storey was watching the race on the big screen in the stadium when Radcliffe was forced to stop.
He told the Daily Telegraph: "We knew where she was but it was traffic chaos out there and we couldn't get to her.
"We knew there was a first aid station at the 35km mark so the simplest, safest and best course of action was to instruct an ambulance from there.
"She was assessed in the ambulance and deemed well enough to come here and so she was brought in to see the doctor."