I trained for my ride in the flats, before buying a Kickr and signing up with TR. This GCN video:
has a lot of great tips for outside riding. It is flat where I live, and we are blessed with strong winds coming from the Bay Area. My training involved one hour rides into the wind, sitting up, in sweet spot and threshold (HR based).
With TR now, for 1-3 hour climbs I would focus on SSB-HV, Sustained Power Build, and either 40K TT or Century specialty. Earlier this year I finished week 5 of SSB1-HV with a 2.5 hour climb in the Sierra mountains. It was a fairly steady 5% grade and all the sweet spot work made a huge difference as compared to my unstructured outdoor prep for the 2016 epic ride.
My max HR is 175bpm, and my “climb all day” HR pace is 135bpm and if I had to give a range it would be 135-145bpm.
If there are steep pitches in the 8+% range then a 36 will give you a break and allow for a more aerobic effort in 80rpm range.