This example taken from a tournament game I played over 30 years ago shows how difficult simple endings can be in practice. The game was never very far from being drawn until White made a small slip at the very end. In these kinds of endings keeping extra P moves in reserve is very important and the ability to calculate a variety of P moves can present a real challenge because you have to calculate quickly and accurately. As I pointed out in the note to White's 40th move, sometimes you don't have to actually calculate; you can just count moves. The ending may look simple, but it was not. I suggest setting the P ending up on a board and playing around with it.