I think intensity becomes a growing “TSS multiplier” as we age. At 51, I can do 4 weeks at 800 TSS and feel pretty decent if most of it is sub threshold. Contrast that with a few 650 TSS weeks that include significant V02 max work and I’ll be pretty shot by the end of week 3.
I’ll get a decent bump from a few weeks of v02 work coming off base, but I can start going backwards from there if I do another v02 block. Just too much fatigue.