It's not less quantifiable than running. It's based on numbers and it's pretty easy to know, within a reasonable range of
hands, how you're doing.
Strict strategy, back testing, analysis, supressed emotions and critical thinking. How else could you tell if you get better or worse?
Find a strategy, put it in play, stick to it, test it over a relevant range. Analyse your play, find leaks, adjust to correct these, if necessary.
Frustrating as it may be, it applies to everything you do in life, whether it's poker, business, crypto, trading, sports, arts, family, relationships, work, love, sex etc.
Your obsession to be the best and beat everyone else at the game you are playing will make you better. Seek information from every possible spot, then test it. If it proves to be right, repeat and consolidate.
Absolutely everything requires hard work in order to attain results. Desired results require even harder work.
If you see the results of your hardest work adding up and your life improving in whatever you do, then you'll know you're getting better.
Not mean in any way, but high rewards require an equal amount of work plus blood, sweat and tears.