I always try to do the intervals, often surprised that I can do a quality job even when I feel zapped before the warm-up. Some of my best workouts are on those days. If you aren't used to the barrage of intervals you can read more into your fatigue than there is - to some extent you should be tired or you aren't pushing hard enough.I’m 8 weeks into a FTP builder plan on Zwift, riding 4-5 times per week. I rested yesterday, but should be bike on the bike today. Unfortunately, I’m dragging. How do you know when to step back and take that extra rest day?