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Olympic champion Mo Farah says he is upset by suggestions he is competing in the London Marathon only for the money.
Farah, 30, intends to run just half of the 26.2-mile course on Sunday.
The Briton said: "That's one of the things that hurts. It's not nice but all the people who know me know I'm not the sort of person who would do that. I genuinely enjoy my athletics."
"Why shouldn't he capitalise on what he did at London 2012?
"At his level of competition one injury could sideline him for many months, could even end his career, and, given the chance, I'm sure most of his critics would do exactly the same.
"He's brought a sizzle to the whole event that hasn't been there since Paula Radcliffe was racing at her best a decade ago."
Farah's fee has not been revealed, but some reports suggest it to be about £450,000.
"I'm not here to just turn out for a bit of money," said Farah, who won the
5,000m and 10,000m
at the 2012 London Olympics. "Every race I do, am I only doing it for rewards?"
Three-time London Marathon winner Paula Radcliffe quizzed Farah's decision to run only half of Sunday's race.
"It's not something I would have done, but that isn't a criticism of him," she said. "I would have preferred to come in to a full race.
"Money isn't the sole reason that he's doing it. My point would be that maybe it's taking a little bit away from the actual race because now the focus is on Mo and where he is going to start."
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“I'm here to learn about the course, so that next year I'm ready to go. It's a no-brainer when you think about it. ”
Hugh Brasher, the London Marathon race director, rejected the idea that Farah was just an expensive pacemaker.
"Sunday is an important stepping stone in Mo's transformation from track to marathon running," he said.
Farah's main focus remains on the track and, in particular, this year's World Championships in Moscow.
But he plans to run in next year's London Marathon and could target Olympic gold at the 2016 Games in Rio.
"I'm here to learn about the course, so that next year I'm ready to go," he added. "It's a no-brainer when you think about it."
Farah was keen to highlight his respect for the rest of the athletes taking part in London.
"I don't want to destroy the race for the normal runners by being at the front and surging, then dropping back and then picking up the pace again," he said.
Farah has also insisted he was happy to compete in London on Sunday, despite the
Boston Marathon bombings.
"This is home for me," he said. "This is where I grew up and there's no reason not to do this."
There will be an
increased police presence at the London race
in light of events in Boston, where three people were killed and more than 100 injured by blasts near the race finish.
London Marathon: Eamonn Martin, the last British male to win
Farah told BBC Radio 5 Live: "Those people [in Boston] would want us to carry on and do the best that we can.
"You can't be thinking about negative things. You just have to be positive and do what we came out here to do."
The London Marathon announced on Thursday
set up to raise money for victims of Monday's bombings.
it would donate £2 for every finisher in Sunday's race to The One Fund Boston,
Race organisers estimate at least £70,000 will be generated, with 35,000 runners set to take part.
Defending champion and Olympic bronze medallist
, 31, heads the men's field with Olympic champion and London Marathon debutant
, 24, and 28-year-old world record holder
also in the race.
With Farah certain not to reach the finish line, 30-year-old Scott Overall leads the British challenge.
Amy Whitehead, 34, is the leading home runner in the women's event, which also includes Olympic champion
, 25, world champion
, 33, and world and Olympic silver medallist Prischa Jeptoo, 28.