How do I develop the self-discipline to persevere through writing challenges?

While I own many books on writing, I've come short of learning to write with excellence, not having practiced most of the skills that are taught in them, and having seldom endured through the writing process. These things are due to my poor self-discipline, which is a serious problem for me, and which long has prevented me not only from writing but also from doing computer programming. People have told me that exercise should help. And some days ago, I tried this a little for a few non-consecutive days. What kind of exercise should I do, and how much should I do, to develop the self-discipline to persevere through my writing challenges? And what other things can you recommend that would help?