The London Marathon will see an eclectic mix of celebrities on the start line, with newsreaders and actors taking their places alongside cage fighters and reality TV stars.
The cast of The Only Way Is Essex is strongly represented with James "Arg" Argent, Cara Kilbey and Billi Mucklow all competing.
Kilbey said she thought they could complete the 26.2-mile course in "about six hours".
The pair, who are raising money for the Samaritans, plan to celebrate their achievement in style.
"We've got a massive party lined up on Sunday night," Kilbey said.
Alex Reid, who is racing to raise money for the British Heart Foundation, said that his love of ancient history meant that he had always wanted to take on the challenge.
"I have always wanted to run a marathon. I knew about the Battle of Marathon and enjoy classical history, so taking part in a marathon myself really captured my imagination," he said.
The cage fighter, who is expecting his first child with fiancee Chantelle Houghton, added: "I am up to 19 miles now in training and it is miserable, although I am enjoying the occasional run.
"I find it very difficult because for 15 to 20 years I trained explosively.
"It's demoralising when old ladies pass you by as you're out."
Other famous faces competing include celebrity chef Gordon Ramsay, who failed to complete the 2010 marathon owing to cramp, and former model Nell McAndrew, running her sixth marathon.
Newsreader Sophie Raworth hopes to "stay upright" after collapsing during last year's event.
Famous figures from the world of sport include England cricket coach Andy Flower, raising money for the Lord's Taverners, who took time out from the team's recent tour of Sri Lanka to train with a 15-mile run through rush-hour traffic in Galle.
James Cracknell, a double Olympic gold medal winner with the British rowing team, is aiming to continue his remarkable record of physical feats by running a third sub-three hour marathon.
The athlete, running for the Children's Trust, has also rowed the Atlantic, walked to the South Pole and recovered from a life-threatening head injury suffered while attempting to cycle, row, run and swim from Los Angeles to New York in under 16 days.
Clayton Blackmore, the former Manchester United full-back, is running for the Rhys Daniels Trust, whilst fellow ex-Wales international Matt Jones is running to raise money for the Spinal Injuries Association, the Bobby Robson Foundation, and the John Hartson Foundation.
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