Just to chip in on your FW target – I just did it in 7h dead after a solid winter of TrainerRoad (Base, Sustained Power Build, Climbing Speciality). I’m 65kg and around 250ftp so about 3.6w/kg. For that time my Avg Power was 178w, so nothing crazy. Training (and the weather) made that possible though, quickest I had done before was 7h48.
If you get near the numbers you’re aiming for, you’d be looking close to ‘winning’ – my riding buddy hits 4.6w/kg and he was 12th fastest on the day.
As others have said, a physiological session with a trainer is golden. I did one with a Guy from BC this spring and it was worth every penny to speak with someone so knowledgable.
Good luck with your goal though, I’ll be watching with interest!