If I'd been cycling long hilly stuff all over lockdown I'd have been looking to do it in about 5.5-6d. Similar to my first time where there was less tracks and about 3,000m less climb; but I did get some long nights sleep then. I was no where near that level this time though.
Depending on your fitness, how much sleep you get and how you handle the tougher off road sections your aim is possible. Don't under estimate some of the smaller Welsh hills on the first day though they can catch you unaware if you are expecting the more classic mountain roads.