You might go the route of either designing your own VO2 progression or using the new TrainNow feature (and later, the progression levels) to select VO2 workouts that are appropriate for you.
I think the built in plans, while good for road racing, crits, MTB, etc., aren’t well suited for ultracyclists and that’s OK (surely isn’t a huge percentage of users here). The events I do aren’t quite “ultra” (6/12hr TTs and non-drafting double centuries), but have similar physiological goals in that I need lots of muscular endurance and ability to ride in Z2 all day long. I’ve ended up designing my own pseudo-plan based on info on this forum (see the Sweet Spot thread for example). For a build phase and especially if you’ve got the time to hammer out a bunch of long distance rides on other days, I think you could use some of the library VO2max workouts successfully. I’m not sure the built in plans will give you an ideal progression if you just want 2 intensity sessions per week.