A typical speed day was 10 x 400 at around 5:40 per mile in substitute for the MLR. This would occur after about a 3 mile warmup, followed by a similar cool down at Z2 HR. In total, I did 2 of these workouts in substitute for the MLRs. I do not think that my loss of endurance can solely be attributed to this... I think there was more of a chronic overload/fatigue problem going on in general.
Regarding the mileage, I agree that it was too much, too soon. About 75-80 mpw would have been a better target. I'll aim for this during the next build. I am sure that I can tolerate that volume after what I just did/mostly adapted to.
After a few weeks of recovery, I also think that doing a short half marathon build for a January race would be a good way for me to keep my fitness and build up my top end without forcing it during a marathon build.