The reality TV stars are competing in the run to raise money for various charities including Macmillan Cancer Support, which helped McQuillan's mum before she died last year.
"When I'm running and I'm knackered I think about the struggle my mother had and it pales into insignificance and gives me a boost," he told the Nottingham Evening Post.
Joking about his fitness levels, the 33-year-old added: "The furthest I have run is to the bar for last orders."
Apprentice champion Siadatan previously competed in a marathon, racing in New York six years ago. Her fundraising will go towards the the Alzheimer's Society.
"My auntie Mary suffers from Alzheimer's and it's very close to my heart," she said. "She was 55 - very young - when diagnosed and she's 61 now. As a family we have been through so much."
Speaking about her relationship with Sir Alan Sugar since winning the show, she commented: "I have gained so much experience by working with Lord Sugar. We have a great working relationship. He's very firm but very fair."