Bah, I'm a J and am procrastinating as we speak. (No worries though, I'm going to put my headphones on in a second and get really focussed for two hours.)
My Jness has caused people to say that I'm stressed out and a bit of a stick in the mud. Not sure you want this.
In any case, isn't the whole point that J/P determines whether you deal with the exterior world through determining what is and what isn't or intaking data? In that sense, it doesn't have to do with self-discipline.
Self-discipline is learned and if you develop your Te then you'll have little more organisation in your life.
I think it would be more about being realistic about what is important to you. If you decide, for example, that having fun with friends is more important than physically keeping in shape, why don't you decide to go out at and play sports with friends instead of focussing on failing at the gym because, in the end, it isn't all that important to you.
I say, make room for what is important and get rid of the rest. It will give you more time and more focus.