Mr Juhari Karim and his wife Nora Ismail plan to run a combined 750km next year for charity.
What is remarkable is that Mr Juhari was diagnosed with Type 2 diabetes in November 2011.
"You cannot let diabetes stop you from living," said the 37-year-old testing and commissioning engineer, who will be raising funds for the Diabetic Society of Singapore (DSS). He was inspired by the exploits of an all-diabetic cycling team from Denmark.
His attitude seems to be an exception among diabetes patients here, according to a survey by pharmaceutical company Bayer, the results of which were released yesterday.
Among the 150 diabetes patients and caregivers polled last month, more than 95 per cent rated their understanding and management of diabetes as "good" or "fair".
But more than 70 per cent believed that they needed to give up a part of their lives - from their favourite hawker food to physical activities - because of the disease.
"Many diabetes patients think they have to give up vigorous physical activity," said DSS vice-president Kevin Tan. "They worry that they feel faint when they exercise."
But that need not be the case with the right controls in place. Mr Juhari, for instance, goes on special diets before and after marathons.
The diets, customised by his wife, a nurse, ensure that he has enough energy to finish the race, but not so much as to get his blood sugar levels too high.
The couple were initially apprehensive about marathon running, given Mr Juhari's disease. But they heard about how cycling team Novo Nordisk, an all-diabetes patients group, took part in one of sport's most gruelling races - the Tour de France.
"So we contacted them and asked them how they customised their diets before and after races," said Ms Nora, 36. "They were glad to help."
Raising the example of Mr Juhari and his wife, Dr Tan said: "Patients can still live well with the right support." Friends and family should eat healthily with patients or exercise with them, Dr Tan added.
Diabetes currently affects one in nine Singaporeans. For those over 60, the figure is as high as one in three.
And numbers are expected to rise as Singapore's population ages, said Parliamentary Secretary for Health Muhammad Faishal Ibrahim yesterday, at a World Diabetes Day event organised by DSS at Suntec City.
This is why early detection is also a crucial step to managing the disease.
"In Singapore, about half of all people with diabetes are unaware of their condition," said Associate Professor Faishal.
Those who are overweight or have a family history of diabetes should go for regular screening, he added.