Marathon runners are at risk for high blood pressure and heart complications because endurance training can stress the cardiovascular system, according to researchers who will present their study at Monday’s American College of Cardiology meeting in Atlanta.

Marathon runners had increased blood pressure and stiffness in the aorta, the major artery to the heart.

Though heart doctors have long encouraged moderate exercise to protect against heart-related problems, lead author Despina Kardara, a researcher from the Athens Medical School and Hippokration Hospital in Athens, has now discovered that exercising too much can be problematic, too.

“The cardiovascular system is like a sports car engine,” said Kardara in a statement released with the study. ”If you do not use it, it will decay, but if you run it too fast for too long, you might burn out.”

The runners in the study trained about 10 to 19 hours a week for about two to 20 years, the researchers said.

Though I have run marathons for three years — usually blowing that 19 hours of training per week out of the water — I’m not changing my ways quite yet. The study included just over 100 people (49 runners, seven of whom were women, and 46 non-runners). And I’m never happier than when I’m training.