Thing is, when you run, or do any type of exercise, you damage your muscles. On rest days, your body repairs the damage and makes the muscles stronger, so next time, you can run further or faster before doing damage. There will come a time when you overdo it, and may pull or tear a muscle. No-one thinks this is too risky. It comes with the territory. The heart is a muscle, so...............
Except that cardiac muscle is unique in the body and unlike any other muscle in the body, although it is arguably a 'smooth' muscle.
Either way it is unlike your leg muscles.
But a lot of this is poorly understood, some people live long despite poor lifestyle choices others die young despite no obvious fault in their lifestyle choices.
This is a work in progress that the press can exploit to make sensational headlines. I am certain that the body has evolved to be active and find that people in A&E are rarely the fit and active........ unless they have injured themselves in accidents or overdoing it of course