A potentially fatal medical scare has put the brakes - for now - on one of Langley's most active senior citizens.

Harvey Nelsen, who will celebrate his 77th birthday on Sept. 3, was two kilometres short of completing the 5 Peaks Trail Run at the Seymour Mountain Downhill Ski Area when he was overcome by exhaustion.

"I had no energy. I thought I was just out of shape, but I said I wanted to finish the last two kilometres," the retired schoolteacher said.

Nelsen went to Langley Memorial Hospital, where he spent 15 hours in the emergency ward. Doctors discovered blood clots in his lungs.

"The doctor told me if I would have gone any further, I would have died. The blood clot would have reached my heart," Nelson said.

On blood thinning pills for roughly the next six months, Nelsen has been told that if he must run in the foreseeable future, he should do so on flat surfaces.

To keep in shape since the July 24 scare, Nelsen has been walking the trails of Campbell Valley and Aldergrove Lake regional parks, and going to the gym.

This is the second setback in the past three years for the avid endurance runner who, over the past 15 years, has taken part in more than 500 endurance competitions, including 52 marathons.

He has done the Sea2Summit Adventure Race nine times, the Knee Knackering North Shore Trail Run five times, Boston Marathon in 2004 and '05, the BMO Vancouver Marathon 13 times, and Beijing Marathon in 2003.

In 2008, a routine rib X-ray following a car crash last summer uncovered a disturbing find.

"Cancer in my right kidney," Nelsen said [Cancer couldn't slow down runner, June 30, 2009, Langley Advance]. "It was destructive to the mind. I was wrapping things up with my family, and preparing my will. It's scary."

He underwent an operation to have the tumour removed in September 2008.

Two weeks after the operation, Nelsen completed a five-hour mountain run in Port Moody.

An MRI that Nelson had last June found him cancer-free. To celebrate, he ran the 2009 Summer Solstice.

Nelsen's most recent marathon challenge was the BMO Vancouver Marathon, which he completed in seven hours, 17 minutes, 13 seconds while competing in the male 75-79 division.

"I took it easy," he said, looking back on his 13th completed Vancouver Marathon.

Nelsen's love of pushing his physical limits is well known in the North Vancouver area. He once completed nine Grouse Grind treks in one day, and his personal record is 64 "Grinds" in a week.

He said, if he is cleared medically, he will take part in a half marathon as part of the BMO Okanagan Marathon Oct. 17 in Kelowna.

He plans to continue to run marathons and trail runs in the summer of 2011.