I do a lot of Vo2 work. And when I do long periods of concentrating on that (4-5 weeks) I see the biggest gains. Then I try to lock it in with a period of sweet spot work, and then back at it.
I don’t know what your regimen is, but that is mine. Vo2 blows the lid off for me. And it sucks just as much for me as it does for most.
I have no history of any sort of accomplishment in endurance sports of any kind. I was a football/basketball player growing up and was pretty good at those. My fastest 5k ever is 21 mins which almost killed me - and I did some triathlons in my mid-30s, but was never able to break 2:30 in an Olympic distance – despite working hard for it.
I am average in every sense of the word from an endurance perspective. I feel if I can do it – anyone can.
The only caveat being that I am really “fit” between 170 - 180 lbs. If you are pretty fit at 220 lbs, I could see that being an issue because achieving a 400 FTP would be pretty special.
Keep at it. Embrace the suck.