That’s a good question. I guess to an extent it will depend on what your personal strengths and weaknesses are.
My weakness is / was longer power so that’s where my training focus was.
I went for progressively longer accumulated time in zone for sweetspot and threshold (not over unders). I mixed it up but generally looked to push out my TTE at the higher end of my Threshold.
The rationale behind this was that whatever my FTP, it was clear that a decent performance on the day would need me to need to operate a lot in that zone due to the amount of climbing involved.
Rather than trying to add 10watts to my FTP (even though it did go up a wee bit over time) my focus was on improving how long I could sustain myself at the higher end of my existing envelope.
At the same time I worked hard on improving my nutrition and slowly cutting excess bodyfat - it’s amazing what some goal-related discipline can achieve 
My weekend outdoor rides were planned to slowly and steadily increase the accumulated climbing and distance - as a consequence pushing out further that time in zone.
I deliberately aimed to hold UNDER my FTP on climbs and just pace them out - a steady hard tap at high sweetspot, then rinse and repeat for the entire ride - so 6 or 7 hours-in, still doing the same.
The end result was I felt accustomed to that type of effort being repeated over a long day in the saddle and was able to replicate the approach across the course of the Dragon event.
Psychologically it meant letting go of the urge to stay with faster riders (which at the start I could have done) at the expense of dipping over my Threhsold more often, which would very likely have meant I blew up at some point.
Interestingly, I caught and passed quite a few people who’d passed me earlier in the day, on the last two climbs, which for me was an (admittedly N of 1) validation of the approach. 
Hope this helps 