depends... at some point we all have 'issues' which come with our 'seasoning'. If they're heart related then definition of the issue/s is important as well as the advice of the 'experts'. Then some form of appropriate monitoring is a good idea... Ultimately, you should have a good handle on 'where' you are and what, reasonably you can do. The 'machine' does best when you ramp up the expectations in a lower, not steeper, level of demand.
I have heart considerations, so I try to keep my efforts within a range which is less likely to bring on consequences. But I still challenge myself, and 'intensity' is a big part of doing my best.
As mentioned. 'recovery' seems essential to building these days - recovery was always important. When young, recovery was a short annoyance. These days it's a good time to stretch back and reflect on the goodness of riding and breathing and recover more/longer... LOL!
I always ride with a HRM, it keeps me aware of myself and keeps me honest.
The body is a variable machine, some days great, other days... well that'll change also...
Be honest and then you can also be demanding of yourself.
Plenty of my zone 3 & 4 and I put my toes into the zone 5 pool often enough - sometimes for a longer swim, when I feel right. And sometimes when with a group, I refuse to get shelled... LOL!
who knows what 'later' brings, but I look to my heros for guidance - Robert Marchand.
Ride On
Yuri