Mo Farah insisted he had unfinished business with the marathon, despite a chastening debut over 26.2 miles in London.
The track great trailed home in eighth place in the Virgin Money London Marathon – almost four minutes adrift of winner Wilson Kipsang, the world record holder from Kenya – as the toughest test of his career proved a step too far.
He failed to achieve his pre-race target of breaking the British record, his time of two hours eight minutes and 21 seconds falling well short of Steve Jones' 29-year-old mark of 2hrs 07mins 13secs.
Farah knew his five global titles on the track would count for nothing against the strongest marathon field ever assembled, and he was right. The 31-year-old was never in contention and suffered in the last quarter of the race, the pain etched on his face.
Farah was not helped by missing a drink stop shortly after the halfway mark, at that point running on his own rather than in a group, with his pacemaker too far in front to help.
Kipsang, in contrast, whose arrival in London had been delayed because his passport was stolen, broke the course record with his winning time of 2:04:29. His compatriot Stanley Biwott was second and Ethiopian defending champion Tsegaye Kebede third.
Farah branded his performance "a bad day at the office", but was adamant he would persevere with the marathon..
"I will be back, 100 per cent," Farah said.
If I could come back in two weeks' time I would. That's who I am. I want to be able to know I can run a great marathon as well as achieving medals on a track."
There was a Kenyan one-two in the women's race too, as Edna Kiplagat sprinted away from compatriot Florence Kiplagat down the Mall to win in 2:20:21
Wheelchair racer David Weir lost out in a sprint for the line in his bid for a record seventh title, beaten by Switzerland's Marcel Hug, while American Tatyana McFadden defended her women's crown just a month after winning a cross-country skiing silver medal at the Winter Paralympics.