Marathon world record holder Wilson Kipsang has warned Mo Farah it is impossible to balance running the marathon and competing on the track.
The Kenyan told the Briton he would have to commit wholeheartedly to the distance at the expense of his track career if he was to have any success at all over 26.2 miles.
Kipsang broke the course record on the way to winning the London Marathon on Sunday, finishing almost four minutes ahead of a forlorn Farah, who trailed home in eighth place at the end of a painful and dispiriting debut.
Farah missed out on his London Marathon debut target on Sunday
The double Olympic champion was adamant afterwards he would "be back" to run another marathon, but Kipsang claimed chopping and changing between the track and the marathon was a recipe for failure.
"You automatically lose, because the training for a marathon and the training for a 10k on the track is totally different," he said. "The rhythm, how you have to run, is totally different between track and road racing. You'll be disturbing the body so much."
There was no chance, he insisted, of being able to win on the track and on the road at the Rio Olympics in 2016.
"No, no, no, no, it's not possible," he said. "The marathon and 10k and 5k, you can not do it. You will not even reach halfway."
Even coming second is now alien to Farah, so accustomed has he grown to winning on the track, and eighth place, so far adrift, would have hurt - mentally as well as physically.
The 31-year-old even missed out on his pre-race target of breaking the British record, his time of two hours eight minutes and 21 seconds falling short of Steve Jones' 29-year-old mark of 2hrs 07mins 13secs.
Vowing to return was an automatic response - "That's who I am," Farah said - but the wisdom of doing so has been questioned.
Brendan Foster, the BBC commentator, said Farah should stick to the track for now, but surprisingly Kipsang encouraged him to focus on the marathon full-time.
"I don't think it is good for him to go back. It's not the best idea," said Kipsang, who thought Farah might have "over-trained" for the race and said the experience would stand him in better stead for next time.
"If he prepares and focuses so much on the marathon he can really improve his time."