I've been interested in this question for some time, largely because I am a middle-aged marathoner with certain chronic health issues. There have been a number of studies on this subject. According to a 1987 report (see NCBI http://www.ncbi.nlm.nih.gov/pubmed/3298928), there were only 36 cases of heart attack or sudden death of marathon runners reported to date in world literature. The mean age was 43.8, running 6.8 mean years at a best standard of 3 h 28 min. 75% of the runners were diagnosed with some sort of heart disease or cardiovascular health risk, but continued running. In 2006, there were 6 heart-related deaths in U.S. marathons (RunnersWorld, http://www.runnersworld.com/article/0,7120,s6-241-285--11405-0,00.html), considered the worst in marathon history.
I do not, as far as I know, have heart health issues and heart disease does not run in my family. However, with a compromised immune system, I cannot consider myself "safe." Therefore, proper preparation during training is of utmost importance. We are trained to monitor our heart rate, even to use some sort of HRM (heart rate monitor) device. The challenge for those of us who are far from physically predisposed to long distance running, because we're late comers to the sport, overweight, returning after a long absence, or have other indicators, is to build our base slowly and gradually.
Physician/geriatrician and veteran marathoner, Walter Bortz, M.D., wrote, "For every person who is overtrained, there are thousands who could and should do more." (http://www.marathonandbeyond.com/choices/bortz.htm). While he supports evidence that running provides "insulation" against muscle and bone decay, Type II diabetes, depression, premature aging and heart disease, and even aging, there is such a thing as over doing it.
Studies continue. Boston Marathon qualifier and cardiologist, Malissa J. Wood, M.D., is more interested in the heart health of participants than in race time and stresses the importance of proper training for such as arduous event as a marathon. The bottom line: "the benefits of exercise can extend all the way to the marathon distance." (http://www.runnersworld.com/article/0,7120,s6-241-285--11405-2-1X2-3,00.html)
My coach has had a heart condition all his adult life. One of our runners had vascular surgery this season. Both still ran and completed our recent local marathon. They are not alone. We seem to collectively believe that running makes our heart stronger. After all, the preponderance of evidence seems to support this notion. The small percentage of deaths reported have been among runners already at cardiovascular risk and who experienced a tragic failure during their events.
Therefore, marathon runners may not be at any higher risk than anyone else of heart failure, unless they are either poorly trained or prepared, or are already at high risk due to mitigating circumstances. One thing I know I do have to learn, however, is how to use my HRM for the best benefit to my own heart health.