I just completed the Trans ALP (MTB stage race in the Alps with 18,000M ascent in 7 days)
Some of the days had 3,400m ascent with 8 hrs of riding. Due to time pressures my longest ride before this race was 2hr40. Longest TR ride was 2hr.
On all the climbs (longest was approx 15k and 1400m ascent I kept my HR in the 130’s bpm (which for me is lower end of sweet spot), this was fine and didn’t “cook” me for the rest of the day.
Take lower gearing than you “think” you need and pace the long climbs slowly (if you have further miles after)
I completed the race at 255ftp and 70kg “reasonably” comfortably.