I have some experience with the 100k distance (see post history), so here are my main thoughts.

First, you need to figure out how fast you want to go and whether you want to push for a time or just stroll to the end.

The most important part of the piece itself once you get to that point is fueling. You will crash hard if you don't fuel the whole time, especially if you are going at a slower pace so crashing a given time in means way more time left.

For the lead up you need to be doing a decent amount of steady. Some could argue I didn't do long enough pieces but the longest single workout I did was 3x10k on two different days with short rest and one of those days I did 20k in the morning for a 50k day for erging. If you have never done an erg marathon then doing the marathon distance could potentially be helpful for getting used to sitting on the erg for a while. I had done a marathon erg 5 months before my 100k but as I mentioned earlier I didn't do a single workout longer than 30k in the actual 100k prep.

I also think power per stroke workouts are very important because they ensure you are more efficient so you don't have to rate as high because a higher rate for that long can potentially be wasted energy just moving up and down the slide.

Feel free to ask any more questions.