Using Comfortable Z2 Pace to Adjust Intensity

Hello all,
I figured that I can go with at %65 average for a couple hours. But this only happens when I am well rested or at the end of a rest week.

While I am at outside of the rest week I can barely hold %55, probably because of remained fatigue from previous intensity days. And actually these Z2 days not feeled as easy as they should be.

Is this normal or it means I am adding too much intensity into my training program?

My FTP is 255 and riding 6 hours in a week and I will boost it to 10 hours per week. Schedule is 2 days of intensity, 1 hours each and remaining hours splitted to 2 days as Z2.

How about adding intensity as just as able me to hold %65 comfortably on Z2 days?