A study published in Mayo Clinic Proceedings and led by Paul Williams of the Lawrence Berkeley National Laboratory and Paul Thompson of the Department of Cardiology at Hartford Hospital, focused on 2,400 heart attack survivors and found that the more exercise they did, the less risk they faced of dying from heart disease, but only up to a point.While benefits to the heart when walking and running are the same, it takes twice as long to burn the equal number of calories when walking rather than running. Benefits began to decline among those who ran more than 30 miles (48 kilometers) per week. Among walkers, the turning point, when benefits were lost, was seen in those who did more than 46 miles per week, or 6.5 miles per day.Even more, there was a statistically significant cardiovascular risk associated with the highest levels of exercise, said the study. Results suggest that the benefits of running or walking do not accrue indefinitely and that above some level, perhaps 30 miles per week of running, there is a significant increase in risk," the study said. "Competitive running events also appear to increase the risk of an acute event."The researchers cautioned that since their study focused on heart attack survivors, their results might not be generalizable to the population at large.