These type of events are pretty much my main focus: my current training plan https://www.trainerroad.com/app/career/bobw/calendar is aiming at next year’s Highland Trail 550 https://highlandtrail550.weebly.com/.
Once you know you can do multiple consecutive long days there’s no real training benefit to doing say a 100 mile ride on a regular basis, maybe one every six to eight weeks just to keep your body used to the pounding but be aware that they’ll take quite a bit out of you and mean a good bit of recovery.
Probably more important is sorting out your feeding strategy. You don’t say if your target events are self-supported, i.e. you can use commercial facilities but otherwise you are on your own, or if there are feed stations.
I do the low volume plans (I’ll be doing Mid Volume polarised after new year) and augment those with just going out for a ride. Those long rides will be Z2, if I can’t chat then I’m going too fast.