Yeah, welcome to the club LOL! It is the same for me if I don't practice them for a while, as they are faster than my natural ability, so I really have to push.WP, thanks for sharing these. I've seen your videos on YouTube but didn't realize you post here.
I'm learning how to play all of "Piece of Mind" on bass too. At this point I can play the first eight songs confidently. I still struggle with "To Tame A Land", which (for me) is probably the toughest Maiden song. I can't play the riff at 4:05, and I always lose steam during the section starting at 4:38. So I can play the first 42 minutes of the album and then fall apart during the last three.
Re the tricky riff, just play it slow until you have muscle memory in your fingers, then speed up over time. I included a video break down of the riff in the comments section on youtube. Good luck!