If you can produce an FTP of 3W/kg, do you do so on the steep hills several hours into a sportive?
Some other things spring to mind.
When you’re plugging your way up a hill, airflow drops and cooling drops. Your body has many innate protections from overheating so if you generally wrap yourself up for your rides, an incline can tip you over the edge and performance drops away quickly. When you’re on a big stretch of a ride like the Fred (without hilly centuries being part of your regular programme) you’ve got a lot to factor in just in terms of managing your stamina. Sometimes you need to go slower to go faster.
The great thing about hills is that you can go up them slowly; going slower moderates the effort within your capability. It can take some practice but, while a hill can give you an urge to strive against it, constantly striving is probably an above FTP effort and you’ll blow quickly like that and actually manage better from doing something that feels like soft pedalling. Low gear. Decent cadence. No surging through the power stroke. No lunging on the bars.
Hills can defeat you mentally before they defeat you physically. You are going slower and it doesn’t feel like you’re going well. That eats away at you. Option A is you respond and overcook it and then blow up. Option B is that your mental game prevents you from just plugging away and you despair and blow up. Option C is that perceived difficulty makes you lose form on the bike and you don’t even perform to your potential.
3W/kg isn’t nothing but also isn’t a big number. You’ve got to be realistic. Your perception of 3W/kg may be “I’m slow”. A plan to grow that 3W/kg will always help but consider mental factors also. Just remember, you’re on your bike; what could possibly be better?