But with anxious millions watching on television, the tears never came - Radcliffe had stopped to relieve herself.
Within a matter of seconds, she was back running on the streets of east London - still ahead of the pack and with her trademark grimace intact - and went on to win the race with a gap of five minutes over her nearest rival.
Speaking soon after the race, Radcliffe apologised for her unscheduled toilet break and revealed that severe stomach cramps had forced her to stop - blaming too much grilled salmon the night before.
"I want to apologise to the nation - I had to stop. I didn’t know how far I was in front," said Radcliffe, knowing television was beaming coverage of her discomfort worldwide.
"I didn’t really want to resort to that in front of hundreds of thousands of people.
"Basically I needed to go. I started feeling it between 15 and 16 miles and probably carried on too long before stopping. I must have eaten too much beforehand.
"I wasn’t getting any information so I didn’t know how far ahead I was."
Radcliffe, who has cut wheat, gluten and dairy products out of her diet since a stomach upset that led to her withdrawal in the Athens Olympics last summer, was again afflicted with bowel problems. "I was losing ten seconds a mile," she said. A headwind also slowed her down, so she was two minutes off her personal best.
She blamed a meal of gluten-free pasta and grilled salmon the night before, saying: "I probably ate too much last night. It was my usual gluten-free pasta and grilled salmon but I probably stuffed too much down me.
"It was nothing like the problems I had last summer, though. It was just that I needed to go, and then I was fine.
"I was glad that the other problems that can be quite debilitating did not occur."
Tens of thousands of people lined the streets to cheer Radcliffe home. Millions more watched on television.
Radcliffe strode to victory in two hours, 17 minutes, 42 seconds on a perfect spring morning, with clear blue skies after a week of leaden skies and rain. Despite her stop, it was the third fastest women’s marathon time ever. It was also the fastest time in a women-only race; when she set the world record of 2:15:25 in 2003 she relied on the help of male pacemakers.
Radcliffe finished more than five minutes ahead of second-placed Constantina Dita-Tomescu, a Romanian, who freely conceded that she had been running for second place.
Asked if she had ever thought she would win, Dita-Tomescu replied: "No way."
In Athens, Radcliffe pulled out of the marathon after being passed by other runners. She also pulled out of the 10,000-metre race.
Radcliffe insisted there had been no danger of her retiring early this time. She said: "It was similar to Athens but the rest of me felt good so there was no danger of me being defeated again.
"Today my body was strong to go with my mind. It was totally different. I had no worries about my preparation or my body.
"In the build-up to Athens I was fooling myself, telling myself I was going to be OK.
"It wasn’t anything to do with expectation in Athens. I pushed my body as far as it could go and it wouldn’t go any further."
Radcliffe knows there will always be a comparison of what she achieves in future to the desperation she suffered in Greece.
She said: "The biggest thing is people won’t let me forget what happened in Athens. I wasn’t right going into the race even though I was trying to convince myself I could handle it. But my body couldn’t do it.
"This is totally different. This was a little hiccup but that was a big hiccup, and I couldn’t get away with it."
Radcliffe’s fifth big-city marathon win earned her more than 130,000, with more to come from her commercial sponsors.
She is now looking forward to this summer’s world championships in Helsinki but is still uncertain whether she will contest the 10,000 metres or the marathon.
Radcliffe said: "That’s the problem - I want to run both. I’d like to win a world championship on the track and also run the marathon."
But there will be no instant decisions about what her competition programme will be until after a lengthy rest.
She said: "I’ll be waiting to see how I recover from the race. It’s 17 weeks before, so there’s plenty of time.
"When I get back into training, we’ll see how my legs recover."