Sounds to me like you're doing the TrainingPeaks thing right. As pointed out, TSS is personal and unrelated to power output. My TSS is currently between 300 and 500, depending on the week. If (when) I start a ride or workout that's supposed to be X and such but, getting into it, I'm just not feeling it after 15-20 minutes, I back it off and take it easy. Conversely, sometimes I do feel it and ride the heck out of it when I'm supposed to be going moderate. I don't always let my forebrain run things for me. It's supposed to be fun.