I think you can get a lot more than 20 years of prime particularly if you take good care of yourself. Even if you don’t take good care of yourself genetics can be kind.

I work in healthcare I see plenty of 50 year olds that there prime ended somewhere in there late 30’s early 40’s I also see plenty of 60/70 year olds who can be argued to still be in there prime. I know a 60 year old surgeon who still runs ultra marathons (50+ km).

Still I understand your sentiment death isn’t what scares me either but the feebleness and pain of old age.