Sir Mo Farah has cast doubt over his long-term racing future and insisted he will know when to quit.

The four-time Olympic champion will run the London Marathon in October, a month after competing in the Big Half in the capital.

They are his only races left this year after he was beaten by shop assistant Ellis Cross at the Vitality 10k in London in May.

Farah insists he still has the desire to compete but conceded he will only know if he can still match the best after London.

He said: “When I line up I always want to do my best, no matter what kind of race it is. Sometimes your body doesn’t allow you, I am getting on a bit. Do I still have the hunger, am I willing to put in the work and the miles?

“Yes. I’ve been putting in consistent mileage and I still have that fight in me. Until you lose it I don’t think I should think about retiring, but being realistic, can your body do this?

“I’ve watched tennis and Andy Murray, the guy still has that fight in him but his body doesn’t allow him. I’m not planning four or five races, I’m planning two races at the minute, then go back and see where I am.

“Can my body compete with these guys at this level? That’s the question which will come afterwards.

“I keep putting the miles in and keep putting in the work. I love to be competitive with others, it’s the reason I’m not going to the World Champs or Europeans.

“If I can’t be competitive with these guys, there’s no point going and making up the team. I’ll give it my all and see what happens.”

Farah returned from a foot injury to finish second in the Vitality 10k, four seconds behind Cross, in his first race for nine months having failed to reach last year’s Tokyo Olympics.

Missing the Games, after attempts to qualify in Birmingham and Manchester last summer, has left question marks over his future, but Farah is adamant only he will say when to retire.

“That decision can only come down from me, not my manager, not my wife or my kids,” said the 39-year-old. “It has to come down from you. It’s you putting in the work week in, week out. There will be a time, but I don’t even know myself.

“It is hard. Honestly, if I go back to before the Tokyo Olympics, I was like, ‘Yeah, still got it, still got it’. But then you get injured and you miss out. You think you’ve still got it because that’s our mind at that level.

“You don’t think anything other than you’ve still got it. But sometimes you’ve got to take a step back, be realistic and let your body do the work.

“I’m kind of learning, taking my time, taking it easy, piling up the miles and seeing what I can do.”

Farah was never in contention for the World Championships in Oregon, which start next week, having spent years training in the American state at the Nike Oregon Project.

He has six world titles in the 10,000m and 5,000m but knew any dreams of competing were unrealistic.

He added: “It would have been nice and as an athlete you love to compete but, again, you’ve got to be realistic. You’ve done the World Champs, you’ve won medals, are you just going there to make numbers or are you going there to actually be competitive? It’s not easy.

“You’ve got to be competitive enough to compete the last 1k in under 2:25. Am I capable of that? Anyone can run 27 minutes, but can you run 26.40, 26.35? I think that’s what it’s going to take to win World Champs.”