Well, where to start…
1.) Planbuilder, while I’m certain a lot of good thought and elbow grease went into it is, to be blunt, nothing more than all the stuff you could find already some 10 years ago here and there with tries to automatically build you a training program out of the barest of restricitions you’d need: a weekly hour allowage for training hours and some idea of what training load you should do (beginner, intermediate, experienced).
It can’t schedule your weekly training load in some meaningful ways according to your preferences (i.e. you have 15 hours but you’d rather spent those on 4 than on 5 days etc.) and it falls woefully short in the regard of longer rides (anything above 2 hours). Plan Builder simply won’t go there. This is in some way in regard of Trainerroads mass clientele which simply can’t be ushered to do anything longer than 2 hours on a trainer and in another Trainerroads own philosophy and focus.
2.) to be fair - their focus as well as their personal interests are in road racing, mountain bike racing (all the shorter, more punchier stuff) and typical TT’ing. Yes, also in Triathlon but I can’t vouch for their Tri Plans as I’m no triathelete. Only lately they’ve gone a bit more on the longer, gravelly stuff. Of course, up to this day still emphasising that you can very much prepare for those also with short stuff. Who will wonder, it’s their selling point.
3.) But to be fair - yes, you don’t have to do only long stuff and you should also pay attention to short workouts working the different energy systems. It would be so nice though if TR would better address the need for stamina building for such long events.
4.) While I normally don’t do Brevets or Audaxes (I went straight into self-supported bikepacking racing) on those Audaxes you tend to find every kind of participant. In my gut feeling you find more of the “What? Training? Training is for wimps!” type of cyclists. Who might shun ideas of structured training and wouldn’t be caught dead on a trainer. But instead have this intrinsic urge to be on the road. If they are on strava, you see them logging 200+ km ride after 200+ km ride… While that isn’t the best time-for-your-money training you can’t deny the fact that sheer volume helps them. Don’t know what was first, though. Their genetic innate capability to just ride without end or the sheer time to spend and lust to ride and then came the stamina.
Bit of both, I guess. If you see what e.g. Lael Wilcox does or what Fiona Kolbinger (last years Transcontinental Race winner) rides seemingly just from their first sitting on a proper bike it makes you a bit jealous. You can’t really train that stuff. Or you can, but it takes you years. Time, spent by those two, would still put them up way in front of you.
5.) TL:DR. Plan Builder is not made for this stuff. I isn’t overly intelligent (as seen and judged by it’s results) and just can work in the very narrow confines of the underlying TR training plans. You can get properly prepared for long stuff and audaxes via sheer volume but also via short and structured workouts adressing your separate energy systems according to your needs. But you have to include longer rides strategically to really build stamina (energy wise, but also mentally and contact point - your behind etc. - wise).