Here's a common scenario:
A talented pianist decides to learn jazz. She learns the melody and chords to a few of her favorite jazz tunes and learns to improvise a little. Then she decides to learn a little more about jazz harmony, and .......CRASH!
All of a sudden everything got unbelievably difficult.
She's told that nobody uses the "simple" root-position 7th chords she's been having fun with. Instead, she reads that she needs to learn "rootless A and B voicings" which contain things called 13's, use "smooth voice leading," and don't seem to be related to any music she's ever played before. Anything less isn't considered "hip."
She diligently sets out to learn the required voicings and does OK for a while. But now it's a struggle to play those tunes she used to enjoy so much. These 13th chords are a lot to think about!
All her initial enthusiasm gets gradually drained as she finds herself sitting down at the piano less and less, until she decides to get serious about jazz "later on, when I have more time."
This is no exaggeration. Its a common scenario that I hear about again and again.
The solution? Build upon what you already know. Can you play "Take The 'A' Train" using 7th chords. Great! Have fun doing this for as long as you like and don't let anybody tell you to do otherwise! If and when you get the urge to explore more extended jazz harmonies, then find a way to learn a little more, but without getting rid of the way you're currently playing the tune.
Maybe you use your current voicings, but you change one note in each to include the 9th. You can learn this is just a few days and it'll give a whole new dimension to your playing! Or maybe you learn to invert your 7th chords for smoother voiceleading, and don't worry about those "advanced" 13s until you're more comfortable with the 7th chord inversions.
You get the idea: Build upon what you already know, in a practical manner. You can even practice those A and B voicings at the same time. But don't stop playing the voicings you already know until you're ready. If you change too much too soon, you'll stop enjoying the process of playing tunes and improvising. Conversely, the more fun you have, the more you'll practice and the more you'll improve. And then you'll be really ready to tackle those advanced harmonies. You'll be ready for them!
You can apply this concept to anything you're learning on piano, in any style. Bass lines, melodic embellishment, soloing, rhythmic complexity,etc. Build upon what you already know and you'll end up going farther. Much farther!
Sign up for my special Jazz Newsletter and I'll send you my ebook on Left Hand Techniques For Jazz Piano