And despite what Capa said, it's fraught with danger to assume that what one of the most talented players of ALL TIME for whom almost everything in chess came completely naturally, will work for you.
Capa pretty much never had to struggle with losing a lot of games at patzer level (us) in the opening and middlegame, so of course he would advocate mastering the endgame.
Go look at games of any player here <1300, and they're making so many mistakes before the endgame and often losing games right out of the opening that you better at least fix most of those glaring errors before even touching the endgame.
I do think all players need to learn the basic checkmates, basic pawn opposition things, but studying the endgame gets difficult - FAST and is NOT for beginners. Something as simple looking as 2 pawns vs 1 pawn is hard enough in certain common setups that IMs have screwed it up in serious tournament play.
I hear this quote from Capa all the time, and I was one of those who took it to heart and tried studying the endgame when I was a 1100 level player here. It was some of the worst advice ever - I still lost ALL my games well before the endgame, and my rating went nowhere.
I'm stronger now, but I'm JUST starting to get to the point where 'real' endgame study is making a difference in my game. That's like 1500+ blitz level. If you're under that, you'll be making game-losing errors well before the endgame in most cases. Heck, even now, when I play 10-min+ games, the (vast) majority of the games are decided well before the endgame. At my level (1550 blitz, which is about 1700ish UCSF equivalent per the 2015 ratings survey) I can see how at this level endgame study will likely be the big factor in advancing my rating from here, but I definitely would say it would NOT be the case in my ratings under this level.
This is coming from someone who adores endgame study - I've been doing Dvoretsky's Endgame manual pretty seriously now, playing out EVERY position against the engine (it's slow going- gonna take me years to get through the book, and I'm sure I'll forget a lot of it on the way!)
All those Russian trainers who like Capa, say 'master the endgame first' are almost all training players talented enough to have a baseline of 2000+, on talent alone. Those players don't struggle like we do in losing games in the opening/middlegame, nor did Capa. You're fooling yourself if you think most people of middling patzer level talent (like myself) can follow the same road and just focus on the endgame at the cost of the opening/middlegame where they're losing the vast majority of their games.
You are an "old man who is going to increase 200 elo soon"?
With all due respect, what you're "going to" do is never evidence of anything. If you actually accomplish your goal, then we'll talk.
+1
But you don't hope to improve, you choose to improve.