is all perspectivei guess. I've noticed my first i is frequently slow, but effort-wise feels ok (i.e. i don't feel like crap). Since it seems like a normal pattern I just go with it unless the second i is bad.
Well, I saved the latter ones only by reducing the distance. There was no way I was running 600's at that speed. This morning's workout saved the week. I'll be careful to build in an extra half day recovery for the next one in three weeks.