How is your FTP derived and how long are you holding it in training?
A number is a just a number, what you can actually regularly do is all that what matters when it comes to the event.
Re: what you should be able to hold in a Sprint its normally higher than you’ve stated imo.
See table…
If you cant or a don’t do 20 minutes or over close to FTP then you aren’t going to in a race (in fact in race shape I suggest you should be able to do 2x 20 at 97-100% of FTP)
Muscular endurance might be your main problem or 260 simply isnt your FTP or your pacing is off, the up hills should slightly offset the coasting, and coasting should be minimal.
^ Good advice about longer threshold and sst intervals.
What are you PLs, without knowing what you are doing in training and what you power durantion curve looks like its hard to know if there is an issue or its unrealistic expectations.
Hope you can give some feedback, I’m intrigued.