During a heavy base building period you will be carrying a fair but if fatigue which is why we caution against expecting great performances during this capacity building period. If your legs are feeling heavy and dead for days on end then you need to back off. The first thing I’d try is keeping a couple of these long Z2 runs down in Z1 and see if that helps. If not you may have upped mileage too fast. Be sure you are doing adequate active recovery between runs. Rolling your legs or swimming will help freshen them up a bit.