Mo Farah's record as a distance runner is close to flawless: four Olympic gold medals, six World Championship golds, five more from European Championships.

Close, but not quite.

Unlike the men who preceded him as Olympic 10,000m champion, Haile Gebrselassie and Kenenisa Bekele, Farah has a marathon-sized gap on his record. In his only attempt, in London four years ago, he finished eighth in two hours eight minutes 21 seconds - far from a disastrous debut, but nowhere close to the debuts of those two Ethiopian greats, not even breaking the British record of Steve Jones set way back in 1985.

Now, track career over, at the age of 35, he is back in London. And there are questions in the spring air.

1. Why should it be different this time?

Bekele ran 2:05:04 on his own marathon debut in Paris in 2014. Gebrselassie led London on his 2002 debut until the 25th mile, and it took a world record from Khalid Khannouchi to beat him. Eliud Kipchoge, faster now than both of them, ran 2:05:30 in his own marathon bow, in Hamburg in 2013.

 

I don't think he should be judged a failure if he doesn't win. I do think he should be judged a failure if he doesn't significantly improve his personal best. And I mean significantly

Dave BedfordRace co-ordinator & former 10,000m world record holder

There is a significant gap back to Farah, who also has a 10,000m best slower than both the Ethiopians. But there is much he learned on that warm April day in 2014, and much he is trying to improve.

"I've been in worse pain in training, and on the track when things aren't going as well, but it was a different sort of pain in London," Farah tells BBC Sport.

"It's really hard to get your pacing right in a marathon. You will have a couple of groups the race will break into, and in 2014, to be honest, I thought I could just work my way through from the back. And I remember thinking, 'I can't go any faster. I can only keep going'.

"The biggest lesson I learned is to stay patient, and that at some point it's going to get tough. And then to try to pick up my drinks. You have to stay hydrated, you have to save as much energy as you can."

No longer working with the controversial Alberto Salazar, Farah is now coached by Gary Lough, husband of Paula Radcliffe, three-time London winner and still world record holder.

"I think when you get into Mo there's always a level of expectation," says Lough. "But I also think we have to remember that we're dealing with a guy who has run 3:28 for 1500m. No-one who's run that time has ever attempted to run a marathon at the very top level.

"There were a lot of things we could change. I wasn't around in 2014, but Paula was, and some of the things she was telling me - I think maybe he was just ready too early.

"We could refine. I think he responds very well to training. We look at the surface he's running on. We look at how he's recovering.

"People have said he overstrode for the track, and possibly he did, but he's very efficient at overstriding. There are nuances around limiting little bits of rotation. It's creating the overall package so that he can perform to the best of his ability on the day."

"Mo has a body which is a machine," says Dave Bedford, elite race co-ordinator at London.

"It is efficient in long-distance running. His motor - his heart, his lungs - tells me he can handle anything longer as well as the shorter stuff.

"Others have said his stride is too bouncy. I think it might actually be a mistake to try to do too much about that. Runners run naturally. I'm less concerned about loss of energy in that stride than trying to run in a way that feels uncomfortable.

"It's a great chance for him to destroy Steve Jones' British record, and for the first time he can post a time that can have other marathon runners fearing him. Because at the moment they don't."

2. Has he left it too late?

Farah turned 35 last month. Bekele was 31 on his marathon debut, Kipchoge and Gebrselassie not even in their 30s.

There is getting round, there is improvement. And there is making a genuine impact. It took Bekele two years of training and racing from that debut to record his personal best. It took Gebrselassie six years.

"We've seen other people of 35 who have run great marathons," says Lough. "I think if the mind and body are still willing, I don't think it's an issue.

"When Mo and I started to talk, his motivation - what he actually wanted to achieve - was something we addressed pretty early. This was by no means a rash decision. It took a lot of talking, a lot of deep conversations, until we got to the point where we were, shall we just press the button on this and go for it?

"If I felt he was just ticking boxes, that he was just doing it because it was expected or because he didn't do so well last time, then I don't know if it was the project for me. But I've seen his motivation and I've seen his desire, and put that together with someone who's immensely talented, I definitely think it's something that will end in a good result.

"With the marathon, if you're invested in it fully… sometimes I think you need to be mentally ready to do it. Some people run their best one on their first ever one, and they can't ever replicate it.

"I think Mo's learned a lot from his first experience, and I think he's learned a lot more through preparation for number two. It's a work in progress. I don't know if we're going to get it totally right first go, but we're going to give it a go."

3. How should he run it?

Up against the brilliant Kipchoge, against Bekele, who has run 2:03, as well as 2017 London winner Daniel Wanjiru, Farah is competing in one of the most stacked fields in marathon history.

In 2014 he could not stick with the lead group. If he tries to this year, at a possible world record pace, it could blow him apart.

"It's a loaded race, so I need to make sure I don't make any mistakes, to save as much energy as I can, but to mix it with the other guys too, not to be afraid of them," he says.

"It's all new for me. I'm listening to Gary and soaking up the miles. I shouldn't get excited too much. It wouldn't surprise me if someone broke the record, because you look at the field. There are so many guys!"

"He should keep them in sight," is Bedford's advice. "He shouldn't get too agitated with them at the start. If it were me, I'd have them within eyesight - maybe 60 or 70 metres at the most, concentrate on how he is feeling, get to halfway and then assess.

"It's a long way. He should be conservative early on. If he thinks he can move up to them in the second half, then by all means do it; don't give up an opportunity. If he's not confident enough to go back up to them, then concentrate on a sub 2:05, and have all of us saying, this guy can cut it at marathons.

"If he goes into this race with a competitor's head on, I think he's got a good chance of outstripping even what he thinks he could do. It might be a foul, rainy London day. Straight away that would change everyone's attitude. The harder the conditions, the better I think he will do."

Eliud Kipchoge, one of Farah's rivals on Sunday, is a double London Marathon champion and holds the course record of 2:03:05

4. What would represent success?

Jones' British record - set in 1985 - is 2:07:13. Farah had hoped to smash that four years ago but ended up only just beating Charlie Spedding's 29-year-old English record.

The athletics world will forgive you one disappointing marathon. Two in a row and real doubts will set in that Farah can ever step up successfully.

"Acceptable for me would be knowing I mixed in with the guys, I fought for it, and that I ran a personal best," says Farah. "That would be a great start for me."

"Mo is a hair's breadth away from being the greatest distance runner we've ever seen," says Bedford. "What I think stops him from being that is that he hasn't broken a world record yet. In London he has to achieve something far more significant than he has so far.

"Can he win it? I think with the field that's put together - Kipchoge, close to being the greatest marathon runner we've ever seen, previous London winner, Olympic champion; Bekele, who has done everything Mo has done on the track but moved successfully to marathon too - makes it a very tough race for him to win.

"Mo really has to focus on whether this is the end of his journey or a staging-post on it. I don't think he should be judged a failure if he doesn't win. I do think he should be judged a failure if he doesn't significantly improve his personal best. And I mean significantly.

"For him to be taken seriously as being on that journey, I think he has to run faster than 2:05:30. Maybe even 2:05. Now that could still be a minute behind the winner. But marathon runners aren't made overnight, and I think he needs a performance that gets him closer to where the top guys are at, in order to knuckle down for the next two or three years.

"I think he'll run 2:05:03. Regardless of what happens in front of him, a time like that would meet every single ambition he has for himself and I have for him. If that was the fastest he ran in his life, then we'd both be disappointed."

Over half marathon distance, Mo Farah won the Great North Run every year from 2014-2017

Over half marathon distance, Mo Farah won the Great North Run every year from 2014-2017

5. Can he be a champion again?

So winning in London in 2018 would appear a stretch. What of London next year, or New York?

What of marathon gold at the Tokyo Olympics in 2020? Big championship marathons are run in a very different way to the major city ones: smaller numbers of runners from the dominant east African nations, athletes being asked to peak at unfamiliar times and in unfamiliar conditions.

"I don't know, I do know I've had a great track career, and I wouldn't change anything, but I know too that marathon is different," says Farah.

"It's about finding out when you hit the wall and what you can do about it. And when you hit the wall, do you do it so hard and there's nothing else you can do and you die - or do you hit it a little bit and continue to run a decent time? At this point, I don't know."

"Mo has time," Bedford cautions, "but he hasn't got endless time. He's probably got a three-year window in which he can satisfy himself that whatever he achieves is the best he can do. By the time he gets closer to 38 he may have retired.

"This is not a victory lap; it's unfinished business. It might take Mo a further year after London, maybe even two, to fulfil his journey.

"If I were a betting man, I would bet against him winning London one year, because the odds would be good. Would I be surprised if he won it? No, because he has amazed me on so many occasions. There is something in Mo that you can't touch. But London will never make it easy for him.

"He's got a far better chance of being the Olympic champion in Tokyo in 2020 than he has of winning London. I would definitely have bet on him winning in Tokyo. That sets exactly where I stand on him.

"And if he wins marathon gold in Tokyo, he automatically becomes the greatest distance runner we've ever seen."