I've read several reports on people damaging their hearts through extreme exercise. Joggers know of the problems like damaged knees or tendons but for some reason, there is a myth that getting your heart to beat faster and for longer periods is doing it good. Everything has a limit though and the scarring in the heart short circuits the electrical pulses and disturbes the normal sinus rhythm. I have heart failure, due to a different condition, but keeping myself within sinus rhythm is important because abnormal rhythms can easily make the heart falure worse. Take it easy/easier is, in my case, something I just have to do.
Not that this means do nothing. It's been a year or so since I was last on a bike ride which left me breathless. After having had several procedure and an ICD implant I was recently cardioverted back into normal sinus rhythm. I have been keeping myself moving by cycling steadily on a trainer in the shed, where I could monitor my heart rate and see it bouncing between 60 and 130. After cardioversion, on the trainer, it just stays short of 90 and is quite steady.
The recent good weather though caused me to venture out into the real world and on both days my heart rate would get upto 120. Good thing is, it quickly goes down again when I stop. Now 120 is nothing for these guys who want to see it around 200 or higher whilst pushing it hard for several hours but, for some people, it starts to get dangerous. The problem is, fit people don't know if they are one of those who might develop scarring. At least I know I have a knackered heart and I am actually quite chuffed to be able to cycle for 2 or 3 hours. Cycling can be for pleasure and enjoyment and doesn't have to be about testing personal limits.