Marathon improves mortality but it seems excess can wipe the benefits or does it?

 But while a study of 52,600 runners showed that pounding the pavement can yield a 19 percent lower mortality rate, that benefit was wiped out for those tallying 20 to 25 miles a week. Not exactly sure how those stats work because we all die eventually.-link

I'm planning to do about 12-15 miles per week as part of my training regimen

 ...As cardiologist Paul Thompson says in the WSJ piece, "The guys advancing the hypothesis that you can get too much exercise are manipulating the data... They have an agenda."
 ...
 Wen and colleagues reply that yes, they do have data -- and it doesn't show what O'Keefe et al. hope:
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We were not able to identify an upper limit of physical activity, either moderate or vigorous, above which more harm than good will occur in terms of long-term life expectancy benefits...-link

It seems there are counterpoints to the arguments for moderation but still extremes should likely be cautiously avoided

Moderation seems to provide benefits with about 7mph being optimal at 10-15 miles per week with less than 7 days of running that is with resting days.

 
Regarding pace, individuals who ran six and seven miles per hour had a significant 21% and 27% lower risk of all-cause mortality, whereas those who ran eight or more miles per hour had a nonsignificant 7% lower risk of all-cause mortality.-link

There may be hope for damaged hearts

 In an act of transformation worthy of any magician, scientists have converted scar tissue in the hearts of living mice into beating heart cells. If the same trick works in humans (and we’re still several years away from a trial), it could lead us to a long-sought prize of medicine – a way to mend a broken heart.-link