Just a casual observation; I’m currently on an 8wk plan (my FTP is correct).
Once a wk i do a group ride outside and always the TSS is more than double any workout on the plan…?
I ‘just’ about complete the harder workouts indoors, so dont think they should be any harder, but somehow manage to cope with so much more outside.
It’s almost certainly the duration. a 2 hour ride is going to have double the TSS of a one hour workout. I think what you see is that the TSS of endurance rides is not much different than the TSS of a sweet spot or Threshold workout and could be even more than a VO2Max workout for a given time period. It’s the intensity that makes those indoor workouts harder.