I've never been able to find any actual literature saying explicitly that extended periods of high (as in >75% LTHR) heart rates are bad for you, but there's some evidence to show that a lot of training over the years can damage the heart.
The fact that a proportion of marathon runners, tested at the end of a marathon, show elevated levels of heart protein similar to that following an infarction, is telling. When I was taken to hospital in Good Friday 2014, my levels were raised - but they said not high enough to indicate a heart attack, "but it looks like you've run a half marathon".
Let's not forget - a marathon is only for 3 or 4 hours, even a 200 is two or three times that.