The heart is the most important muscle in the body, and more than a half century after Dr. Kenneth Cooper's recommendations to train the heart via aerobic exercise -aka cardio- there continues to be evidence that this is not the most beneficial way to improve heart health or healthspan. This study looks at the benefits of increasing skeletal muscle mass specifically on heart health.
The study found participants in the highest skeletal muscle mass tertile (the people with the most lean muscle mass) had 81% (HR=0.85) lower risk for a CVD event (heart attack) as compared with those in the lowest SMM tertile.
One way to look at it, despite all the extra time spent running, it is sprinters who likely have healthier hearts than distance runners when muscle mass is factored in.
So even if you enjoy running for the pure joy of it, consider rebalancing a little more time to the weight room, your heart will appreciate it.