Not sure if I’m missing something, but none of the SSB LV2 rides are 120 minutes. Mary Austin -1 and Leconte are “only” 90 minutes? Still, 90 minutes are the long rides on the LV2 plan, so perhaps the issue is you just haven’t built up enough fatigue resistance? Or, maybe aren’t fueling properly. I can crank out basically all the 60 minute rides fasted with no food, but not all of the 90 minutes rides, and especially not these capstone rides. It’s important to know which rides need serious nutrition considerations for you.
I’ve pretty much just finished these rides in from the SSB MV2 side, so they’re pretty fresh in my mind. The threshold/overunder workouts are definitely draining, both from an intensity perspective and interval duration. My take was Spencer+2 was way easier than Mary Austin -1, which was pretty hard, but not as bad as the reputation is seems to have acquired would suggest. Lamarck and Leconte were both much harder, but I never thought about adjusting the intensity, either. If you had to drop MA-1 to 90% that does not bode well for Lamarck or Leconte.
I personally found it helps a ton to have the even longer 2hr sweetspot rides with intervals in the 15+ minute range, which makes the threshold 10 minute ones just fly by in comparison. Since you’re on the LV plan, the 2hr rides are probably a stretch, but maybe take a timeout this week and do some sweetspot workouts with 15-20 minute intervals in the low/mid 90%. For example, Eichorn, Wrangell -2, Mount Goode -2, or Rainbow -1.