KRpKR rookendings

Now my games become longer, I get rook endings on the board. To understand rook endings, the first task is to get the hang of KRp vs KR endings. The Philidor- and Lucena positions form only the beginning of this type of ending. One has to learn to make progress when the pawn is still on the fourth rank all the way to promotion. To learn that from books is a very daunting task.
Virtualy impossible.

I made a filter at CT with 1600 KRpKR problems. That should be enough to get the hang of it.
I have done the first 100 problems, and I scored 70%. Of course that must become 100% before I can move on to the next stage. What emerges is a general method to make progress towards promotion. Already I approach that systematically. But every now and then, there is a special element in the position that prevents such straight forward approach. At first sight it can appear to be rather esoteric.
By doing a lot of exercises, I hope to discover some similarities in the positions that aren't straight forward. Sofar the amount of problematic positions seems to be fairly limited. Yet it is difficult to wrap my head around them. Take for instance the following position:

Black to move.
Only one move leads to a win, thirteen moves lead to a draw and five moves lead to a loss.

Winning move [ Ra2+ ]
Though the process is slow, the more exercises I do, the more clarity emerges. Learning this from a book I consider to be too daunting to be realistic.