Dr. Crawver said:
Surprised Jeet Kune Do hasn't been mentioned yet. Being built entirely around efficiency and adaptability, there are definitely styles that get the job done faster (it being more reserved than some all out techniques), it should be able to deal with whatever comes at you. It is however painfully hard to learn as it's as much a mindset as a list of moves.
It's my understanding that Jeet Kun Do is a basis for many of the modern military styles, so was also surprised that no one's mentioned it.
@OP I started JKD for similar reasons to what you've explained. The main concern I had was finding a physical expression for my zen practice; to practice calmness in activity. Basically I needed something to practice focusing, clearing my mind while gradually and mindfully building my body to meet the demands of the practice.
What JKD is.
It's a fighting style developed by Bruce Lee. He believed that, as has been pointed out in this thread, the problem with "martial arts" was the arts part. He criticized traditional styles like karate for having become like sports and having lost the true essence of combat.
What it did for me.
So I got Bruce Lee's Tao of Jeet Kun Do. Basically his notes were published after his death. So I read the words, looked at the pictures all the while trying to understand the "essence" of what I would be trying to accomplish.
Bruce Lee constantly reminds you that Jeet Kun Do is a basis into which you can add and adapt almost anything from any style. He stresses that your practice is a personal/emotional/spiritual thing as much as it has combat-motive. With the wrong mind you will lose every fight; internal or external.
I approached JKD as a self-learning exercise and after a few years of practice I have a limited, but sufficient, range of "moves" that I practice mindfully whenever I am alone, or sparring with a friend. My kicks are quick, accurate and powerful. My hands are strong and I harden them constantly. At around 186cm tall, weighing around 72kgs, I am slim but strong and flexible; I rediscovered the joy of movement, something that I believe most of us lose when we're no longer children. I am confident that I'm able to execute correctly and with the right mind. Though winning a street fight depends as much on my opponent as it does me.
Practice was never suffering and there was no feeling of "Meh, I don't want to do this today.". Gradual, mindful and constant practice of basic principles.
Don't get discouraged.
Here's where I have to get a little zen. Whatever OP chooses, the easiest way to become discouraged is to have goals in terms of expectations. Expectations never meet up with reality and failing to meet them can easily discourage you from practice. Having an expectation of your practice is a little like asking "What does this offer me?" and then answering your own question when the answer is actually "I don't know yet, I haven't been practicing it long/diligently enough to realize.". You do what you can do and the rest will take care of itself.
Balance, mindfulness, strength, agility, stamina, speed. It doesn't matter how you practice as long as you're not sliding backwards.