Long races are never safe. Running in today's world, where we spend most of our time sedentary then suddenly go out and do one burst of intense exercise and mostly run on pavement instead of barefoot on soft earth or grass, of course it isn't safe. But most of what we do isn't safe. Driving isn't safe. If this is what you love, then do it. Do it as safely as you can, but your body is your body -- some people crash early in life, some go go go until their 90s. If you're one of the latter, just consider yourself lucky and enjoy it while it lasts. Now, if safety is what you're after, running isn't ever safe -- forget about your heart, what about your knees and your plantar fascia and your hips etc. etc. Now, as for the heart, there is research that shows marathon runners have a higher rate of cardiovascular disease than others. But you're not running marathons. The issue seems to be the stress point where the body is no longer running on nutrients but instead starts eating itself. There's also the inflammation issue. But again, if this is what you enjoy more than anything, do it but keep watch on your health as you do.
Do you know your resting heart rate?
Do you have any health conditions and take any meds?
Exercising above 85 percent of your maximum heart rate could put you at risk for over-training. If you over-train that could cause your body to get weaker not stronger, that can lead to increased fatigue and decreased performance. You also are more prone to injury and illness when you over-train.
The Karvonen Formula is a mathematical formula that helps you determine your target heart rate zone. The formula involves using your maximum heart rate (MHR) minus your age to come up with a target heart rate range (which is a percentage of your MHR). First thing in the morning before you get out of bed have a clock with a second hand and check your resting heart rate then figure your rate by the Karvonen Formula
If you are taking medication then proceed with caution, but if not take 800 milligram of magnesium daily to allow the heart the contract and relax properly. I also suggest you take liquid oxygen to prevent the heart from fighting to get air.