Consistency is HARD

The TR workouts often ask that question - will pushing through this workout affect the next workouts performance? Well my response is - HTF do I know? I just do it and then see tomorrow if I can get through that workout and just keep going. I have yet to fail a workout but I am suffering through them lately. Interestingly its not my cardio that is the issue but my muscle strength/endurance. These 2 hr SS sessions leave me with the feeling I am tearing my thigh muscles to shreds (quads in particular) while my HR barely hit LTHR by the end of the last interval.