EVERY time the mail drops though the letterbox at home in Aviemore, David Smith prepares for the worst news. He immediately identifies envelopes from the hospital, and knows it may contain a death sentence, or potential paralysis from the neck down. "It is a time bomb in your hand," he says. "You feel healthy, but opening it is one of the toughest things. Your whole life is in your hands."

Smith is a 2012 Paralympic rowing gold medallist, and training to return to the British team for Rio de Janeiro helps, it "keep the demons at bay". A tumour the size of a tennis ball has been removed from his spine, briefly leaving him paralysed. He had to learn to walk again. Then the tumour returned, and he had to endure it all again.

Surgery weakened his arms, and he can't row. Undaunted he now hopes to make the GB cycling team for Rio.

Even in the hyperbole-infused world of elite sport, where inspirational triumph over adversity is routine, Smith's story transcends exceptional. His mind-set is literally death-defying. This is matched by a predictable zest for life, and the elite sportsman's drive and positivity, yet also a humbling philosophy.

David was born with club feet, facing backwards. The bones were surgically broken and reset in plaster boots. The alternative was below-knee amputation.

Despite his right foot remaining fused and deformed, David took up karate. His potential was dismissed but he gained a black belt and was in the British squad for six years. "I dreamed of going to the Olympics, but karate is not an Olympic sport," he said. So he took up sprinting, and became East of Scotland 400m champion in mainstream athletics, and took third in the 200m behind Olympian Ian Mackie. But running round bends caused stress fractures which forced him to quit.

He turned to bobsleigh, because straight-line running was fine. He was in the GB team as brakeman. But neck and back pains interrupted training and he missed a 2006 Winter Olympic spot by one-hundredth of a second.

Alerted by a physiotherapist that his foot condition qualified him for Paralympic sport, he turned to rowing and discovered outstanding aptitude. Within five months he was in the GB crew which won World gold.

The Olympic dream beckoned again, but then the tumour was discovered. "It is genetic. I probably had it in the womb," he said. "I've discovered this only in the past few days."

After the tumour was removed he had to learn to walk again. "Nothing can be as hard as waking up in a hospital bed and not being able to move," he says.

Yet he won gold in London 2012, in the mixed coxed four. Hopes of defending in Rio next year were dashed by neurological issues in his arms. So he switched to cycling, and had several top-three finishes in mainstream time trials before a routine scan showed the tumour had recurred.

"I wanted to wait until after Rio, but was told if they did not operate, I'd be paralysed from the neck down." He underwent successful surgery last October, and while in hospital set himself the target of cycling up Mt Ventoux, where British cyclist Tommy Simpson died on the 1967 Tour de France. He felt that would indicate he was ready to go for Rio.

His progress has been followed by cameras including a video diary: through months of rehabilitation, through pain, tears, solitary introspection, physiotherapy, bleeped expletives, and three times up Mt Ventoux in one day. This provides the basis for a compelling and powerful documentary, Dead Man Cycling, being screened by BBC Scotland on Tuesday [Aug 4] at 9pm.

"It was hard – very intrusive, when you're full of morphine and raw emotion," David told Herald Sport in an exclusive interview. "But I kind of want people to see what it's like. People always see me happy, smiling, and out on my bike, but I cried quite a lot through it. There were lots of lonely moments when you think it's still in there – can't remove all the tumour and if not, will I die? It's turned into a race, almost. Everything I try to do is stop the tumour growing back. Since the programme was made, I have been told there's a 68% chance of recurrence. It doesn't respond to radiotherapy, or chemo. Only surgery. So you have to live every day to the max. I do not take one minute for granted.

"Every day I wake up I and say what I am grateful for in life [a tip from former Manchester United keeper Gary Bailey]. I do that night and morning, and it's had a huge impact. I face the future with gratitude, go forward hoping no scan comes back positive.

"I live my life in five-month blocks, between scans – it's the only way. I am never going to ask why this happened to me. I try to take the positives from it, and hopefully if my path crosses other people, they can take strength from that."

He has a degree in sports science, but jobs have mainly included labouring with his dad's business, and working as a bouncer. "The best job was as a fitness instructor and assistant coach with the British ski team." His pals include sprinter Mackie, and Aviemore skiers Alain and Noel Baxter, and Finlay Mickel.

"A lot of my mates are Olympic athletes who have a strength. I have drawn on this. They work hard, and are good at pushing me. They're all very positive mentally. Usually if you are ill, you're surrounded by other ill people. I'm surrounded by highly driven athletes."

He is inspired by Graham Obree, the former one-hour world record-holder, whose metaphor for time trials appeals to Smith: "Like sticking your hand in a flame and seeing who lasts longest".

David admits to having a bucket list. "I want to see the world. Sport has given me the possibility to travel round the world, but I would like to see it without the pressure of sporting competition. When I was on my back in hospital, I thought of so much I wanted to see. I love photography, and I know what it's like to face losing my life. I thought of the beauty of the world we live in, and how I'd like to photograph it. And there is a guy 10 years older than me, in my category, who wins a lot. So if I stay healthy, I am looking at Tokyo in 2022.

"Going to London 2012 was an obsession. If I get to Rio it will be a dream come true, but being alive is now a bigger goal."

For more than 10 years doctors failed to diagnose why he had recurrent pain, blaming his sporting lifestyle, and lots of driving. "What ultimately led me to Para-sport was my foot. I'd never have met the physio who diagnosed it and never have found myself in the place where such treatment was possible. I think I've been lucky.

"I know I can get to Rio if I'm willing to suffer on the bike."

David Smith knows he can do suffering.