In July 1984, the famous author and runner Jim Fixx died of a heart attack while running. One lesson from his death: just because you run does not mean you are safe from heart problems.Thirty-five years later, Boston Marathon race director Dave McGillivray is repeating that message. "Being fit and being healthy aren't the same things," he says.McGillivray, a lifelong runner, should know. Six months ago, he had heart surgery after suffering chest pain and shortness of breath while running.McGillivray's family had a long history of heart problems. "I honestly thought that through exercise, cholesterol-lowering medicine, good sleep and the right diet, I'd be fine," he says. "But you can't run away from your genetics."Aerobic exercise such as running, walking, cycling and swimming is known to reduce the risk of many health problems. But new research is questioning the value of "extreme exercise."In a study published in December, researchers in Spain found information suggesting that full marathons, such as the famous Boston marathon, might injure the heart.Dr. Kevin Harris is a cardiologist. He notes that running is a good activity, but the act of running "doesn't make you invincible." Harris adds that people who have a family history of heart problems should talk with their health care provider before running a marathon. "Especially," he added, "men who are older than 40 and those people who have symptoms they're concerned about."The Spanish researchers, led by Dr. Juan Del Coso, found that only about one in 50,000 marathoners suffers cardiac arrest,a medical term for when the heart stops beating. But, a high number of cardiac events that are caused by exercise happen during marathons.The Boston Marathon, and many other major races, place defibrillators along the race course. Defibrillators are devices that use electricity to shock the heart.McGillivray says his doctor allowed him to race in the upcoming Boston Marathon. He has competed in it every year for 47 years. "My new mission," he says, "is to create awareness: If you feel something, do something... You might not get a second chance."