Fireman John Robson, from Durham, is a veteran runner with many marathons and half marathons under his belt. He has also climbed mountains around the world.
In 2013, he was due to complete his fifth London Marathon, and then set off on an expedition to the Himalayas, but instead, he faced an even greater challenge.
He lost three stone in weight in four weeks, and became weak, tired, and in severe pain. Within weeks, he could not even get out of his bed.
Eventually, John, 51, was diagnosed with a condition called Polymyalgia Rheumatica, an inflammation of the arteries, which normally affects women in their seventies.
"On the London Marathon day," he said, "I couldn't even raise my arms. It was about 6:30 in the morning and I said to my wife Ruth, 'I can't even lift my arms now'. That came as a big shock.
"It took two or three hours to get my out of bed and downstairs, and then we switched on the TV, saw the runners, and the music hit us. It was very emotional."
"I was beyond worried," John's wife Ruth said. "All of a sudden you've got someone who's fit, active, and got a real outdoors-y life, and then you think the worst, imagining myself a widow with two children, and just simply feel beyond worried, absolutely frantic."
John began a programme of steroids and has gradually recovered, taking his challenges step by step, although he will live with the condition forever.
This year, he has decided he is well enough to tackle the Great North Run again, although he has not yet managed much more than an hour's running in training so he does not know whether he'll be able to finish.
"My son said it's like my body has been frozen in time for 15 or 16 months, and that is what it feels like,'" John said.
"I'm not angry. I've got away with it, there's a lot of people worse off than me. We collect money for charity and I see this as a chance to raise money and awareness of this illness. I'll run with a smile."