An Ontario man who was given little chance of survival after a heart attack left his heart working at a quarter of its normal capacity is wowing his doctors nearly 30 years later -- by running partial marathons.

Medically speaking, David Lawrie shouldn't be alive. At age 42, he suffered three heart attacks in a row, destroying 75 per cent of his heart.

"I didn't know what it was. I just couldn't breathe. I fell to the ground clutching my chest," Lawrie, 69, told CTV.

Before the heart attack, Lawrie, a Milton, Ont. car salesman, had never been sick. Now he was fighting for his life.

"I heard (doctors) in the hall that there wasn't much more they could do for me," he said.

"I told myself, ‘I'm in God's hands now.'"

Lawrie's heart attack -- the result of a blocked artery -- was as bad as they come. His doctor warned that even a sneeze could trigger a fatal heart attack.

His weakened heart muscle was working at less than a quarter of normal efficiency. Doctors prescribed medication and gave him an implanted defibrillator in case his heart stopped.

But even his doctor, Manish Maingi, had his doubts.

"People that have severe heart dysfunction don't normally live a long time," said Maingi, chief of cardiology at Credit Valley Hospital.

He described Lawrie's stricken muscle as a "bad heart," one that does more "twisting and jiggling" than beating.

Lawrie wondered how much longer he would survive. "I said: ‘Do I have a future?' And they said, ‘Take it one day at a time.'"

That was nearly three decades ago. Since then, Lawrie defied his grim prognosis, surprising doctors with his perseverance, positive outlook and staying power. He hopes his high quality of life post-heart attack inspires others.

The father of six resumed his busy life, had two more children and returned to his job selling cars, often logging 12-hour days.

He began to exercise, gradually, at first. Then, in his sixties, he decided to run a marathon.

"Wouldn't it be something for a guy with a little bit of heart pumping to run a marathon?" he asked. "Who would ever think that a guy with a little bit of heart pumping could finish a half marathon?"

Dr. Maingi tried to discourage him.

"I appreciate his enthusiasm. We spend time trying to motivate people. He's someone I spent time trying to de-motivate because I worried that would be (an) excessive strain on him."

Lawrie defied his detractors, running three partial marathons all in his sixties.

Maingi said his patient's good health shows that the right attitude can trump a devastating prognosis.

"There is something, willpower, motivation, perseverance that we don't measure with numbers, and that makes a difference," he said.

"You can't underestimate people. Psychology is a huge part, psychology and attitude. In Mr. Lawrie's case, that's played a huge role. He's done stuff that a lot of people, a lot of experts would say is impossible."

With a report from CTV's medical specialist Avis Favaro and producer Elizabeth St. Philip