McColgan, who finished a disappointing 16th after losing touch with the leading group early in the race, had an insect bite on her right heel which was bandaged. She was on antibiotics and was sick the night before.
"I knew it was all over after three kilometres," she said. "My glands are all up. I'm very angry. After 19 miles I was going to drop out but I thought I would have to wait too long for the bus."
She said there was no question of her retiring and would probably run the Tokyo marathon later this year. "I'm so annoyed. I can't go out like this."
Her injury-plagued career had brightened earlier this year when she won the London Marathon but here there was no chance of reacting positively to the ultimate test.
As in the London Marathon she was quickly involved in a dilemma; whether to make an attempt to catch the breakaway favourite, Uta Pippig of Germany, or do as she did in London and think tactically. Here, though, she had no reserves of strength. Pippig's pace was too great and her own was fading.
Pippig had led the field out of the stadium in drizzle and with daylight only just breaking when at least the temperature was only a little over 70 degrees. Nevertheless, the humidity was high. She soon took a 25-second lead, but that proved to be a bad misjudgment and her smile gradually turned into a grimace. A chasing group consisting of Valentina Yegorova, of Russia, Yuko Arimori (Japan), Lidia Simon (Romania), Maria Machado (Portugal) and Katrin Dorre (Germany) slowly eroded her lead.
By 10 miles Pippig's lead was reduced to 15sec. But Dorre had dropped out of the chasing group. A mile later Pippig found herself engulfed and slid into no-medal land as the race moved into downtown Atlanta.
Roba's decision to attack after 12 miles was perfect timing. Within another mile she had accumulated a 17-second advantage. Lithe and long, she seemed to glide through the miles, passing old men sitting on their verandas nodding gently from their rocking chairs and wildly enthusiastic flag- bearing Ethiopians to whom she waved: the salute of a copper-haired woman who had gold in her eyes.
There was still the gentle but frequent climbs between the skyscrapers and the equally muscle wrenching, long downhill section, but her lead kept growing. At 16 it had grown to 45 seconds and McColgan was struggling far behind. Pippig, too, was suffering.
Arimori made a brave attempt to cross the gap which by 18 miles was over a minute. Yegorova watched from a distance knowing that a steady pace would probably bring her back into contention for silver. She was right. Ahead, though, Roba, had such a superior lead that she could enter the packed stadium dramatically, running a full lap entirely alone before Yegorova's arrival followed by Arimori. It was Roba's third marathon win this year.