I don't know how old you are or your fitness. I'm 63 and do a century a month. I live near lots of beautiful hills and love climbing them. My max HR is 190--I know because I hit it every once in a while. I listen to my body more now than when I was younger. I will sleep in 5-6 times a year and change a hard morning ride for an easier one if the body says so. I have no heart or health issues but I ask the same question you do. I still train hard and push myself to the limit because I'm afraid if I don't, the limit will decrease. I've heard of fit cyclists having heart attacks. I say listen to your body, but don't be afraid to tell it to shut up every once in a while because some of my best rides have been when I thought I was tired and I surprised myself.