I thought you might be interested in the following letter from Erin Karlstedt. If you do decide to participate I guarantee you'll get THE BEST medical attention our system has to offer. If there's something wrong with your heart they will find it. It's a great opportunity to rule out any concerns.
I blogged the experience here.
Good Afternoon 2010 Marathon Study Participants,
Dr. Jassal and I are conducting another Marathon study at this year's upcoming marathon. We are hoping to acquire more data for your age group (50+) to strengthen our results before publishing our paper.
We are in desperate need of additional participants and are hoping that you can help. If you know of any friends or colleagues who might be interested in participating in our study this year we would greatly appreciate it. They can contact us at any time with information provided below.
I hope I find you all doing well. Thank you again for participating last year, it was a great success!