Mr Roberts, 35, is running to raise money for the Meningitis Research Foundation (MRF).
He said, “My wife and I met Kayleigh and Phillip Russell at RAF Akrotiri, a Royal Air Force station in Cyprus.
“Sadly their 18-month-old little girl, Claudia, otherwise known as CJ, died very suddenly from meningitis in 2016.
“She was just six weeks younger than our daughter Lily and the girls were growing up together until CJ’s life was cut short.
“We want to play our small part in keeping CJ’s memory alive by raising money for MRF.”
Mr Roberts joined the army as an ammunition technician and is now a qualified bomb disposal expert who trains fellow servicemen.
He has previously done a half marathon in Cyprus, and the Great North Run but he has also broken his ankle three times leaving him with a permanent limp, stability issues and constant pain in his calves and shins, meaning London will be a big test.
Vinny Smith, chief executive of MRF said, “We’re really grateful to Phil for taking on this huge challenge to help raise funds for MRF. “Meningitis and septicaemia can strike quickly, sometimes killing in hours, and leaving some survivors with life-changing after effects. “The money raised will enable us to fund vital scientific research into the prevention, detection and treatment of meningitis, raise awareness of the disease and support those affected.”
The first symptoms of meningitis and septicaemia are usually fever, vomiting, headache and feeling unwell.
Limb pain, pale skin, and cold hands and feet often appear earlier than the rash, neck stiffness, dislike of bright lights and confusion.
People who are ill with meningitis and septicaemia can deteriorate rapidly and not everyone gets all of these symptoms.
It is important for people to trust their instincts and get medical help immediately if they suspect meningitis.