Where to start and how will it end?

So here I sit, a sub-1600 OTB player trying to come back into chess pushing 45 years of age and after being away from serious chess for 10 years. I have to confess that the task before me is quite daunting, if not entirely intimidating. I am asking my aging, damaged brain to function like a precision machine within this unique world.

And my brain is damaged, I had a fairly bad car accident a few years back where I took a blow to my brain severe enough to give me trauma induced epilepsy. While I am now seizure free with no serious concerns for relapse, I do have recuring issues with my mental function, ranging from some difficult emotional swings and problems with memory and vocabulary.

Yet I remain largely positive about my prospects to experience signficant improvement at this game. I have no illusions that I'll be a GM some day, or even an IM, I do wonder what my upper limit is? Where should I set my sights that would be possible for me? And how do I go about achieving those goals?

Right now I'm working with an IM doing weekly lessons working on the basics of the game, and going over my own games to see what I'm doing well and what I can learn from. I'm working tactics problems regularly on http://www.chesstempo.com. I'm watching videos on various topics. And lastly I'm seriously and judiciously working through Artur Yusupov's Build Up Your Chess series.

But for all of that, when I sit down at an OTB game I am still playing well below my peak rating, which was a very sub-par 1480 many years ago.

I plan on adding annotating and analyzing master games to my routine. I've read Dan Heisman's The Improving Chess Thinker and I recognize little of myself in the words of the higher rated players. I do think I can build the discipline necessary to move forward to become a better over the board analyst, which seems to be a trait of the expert level player.

I know that I'm not alone or unique in my dreams and goals. But I do wonder how many of us seeking real improvement as adults actually succeed? What sets those who do succeed apart from those who don't? What is the key to being one of the few who step up their game to the next level? Desire? I have that. Dedication? It's there as well. Hard work? Obviously that's something we all need and I do. Is it just being lucky enough to do all that and have one's brain be pliable enough to still learn a new trick?