The longer the race, the more critical it is to have medical clearance, as Associated Press reports.

It was the death heard 'round the running world.

In July 1984, acclaimed author and running guru Jim Fixx died of a heart attack while trotting along a country road in Vermont.

Overnight, a nascent global movement of asphalt athletes got a gut check: Just because you run marathons doesn't mean you're safe from heart problems.

Fast-forward 35 years, and Boston Marathon race director Dave McGillivray is amplifying that message for marathoners, especially those who have coronary artery disease or a family history of it.

"Being fit and being healthy aren't the same things," McGillivray said.

He should know. Six months ago, the lifelong competitor underwent open-heart triple bypass surgery after experiencing chest pain and shortness of breath while running.

As marathons, ultramarathons, megamile trail races and swim-bike-run triathlons continue to explode in popularity, doctors are represcribing some longstanding advice: Get a checkup first and talk with your primary care physician or cardiologist about the risks and benefits before hitting the road.

For McGillivray, 64, the writing was on his artery walls. Both of his grandfathers died of heart attacks; his father had multiple bypasses; his siblings have had heart surgery; a brother recently suffered a stroke.

Neither McGillivray's marathon personal best of 2 hours, 29 minutes, 58 seconds, nor his decades of involvement in the sport could protect him.

"I honestly thought that through exercise, cholesterol-lowering medicine, good sleep and the right diet, I'd be fine," he said. "But you can't run away from your genetics."

Aerobic exercise such as running, brisk walking, cycling and swimming is known to reduce the risk of heart disease, high blood pressure, stroke and certain types of cancer, and it's been a key way to fight obesity, Type 2 diabetes, osteoporosis and more.

Studies have shown those who exercise regularly are more likely to survive a heart attack and recover more quickly than couch potatoes.

But new research is providing a more nuanced look at "extreme exercise" and the pros and cons of running long.

In a study published in December in Circulation, the journal of the American Heart Association, researchers in Spain found signs suggesting that full marathons like Boston can strain the heart.

They measured substances that can signal stress and found higher levels in runners who covered the classic 26.2-mile (42.2-kilometer) marathon distance compared with those who raced shorter distances such as a half-marathon or 10K.

Only about one in 50,000 marathoners suffers cardiac arrest, the researchers said, but a high proportion of all exercise-induced cardiac events occur during marathons - especially in men 35 and older.