Discipline is doing what you are supposed to do, not doing what you are not supposed to do and pushing yourself to make real progress. As writers we must discipline ourselves to write, and write well. It can be difficult to chain yourself to your work, but the rewards are inestimable.
It is hard to create prose: to sit in one place, staring at a screen, struggling to make sense of all the thoughts in your head. Exercise is exhausting, whether physical or mental. Writers must read to keep their perspectives fresh. They need a strong vocabulary to articulate ideas. And they should constantly be struggling for improvement: to make this story even better than the last.
In the gym, one struggles against seemingly insurmountable obstacles… and overcomes them. It can take days to see progress. And it can be so easily undone. So, writing is contending with words and must be approached with the determination that you will succeed.
I will not go as far as to say that lesser forms of entertainment are akin to junk food for the brain. However, to improve in writing, it is best to read. And the best things to read are those that have stood the test of time: the classics.
To be strong, one must follow those who are. There is a reason we still read certain authors’ works sometimes many years after they have died. The language may have changed, but that which compels, that which reaches the human heart remains constant.
Mental muscles, too, can atrophy from disuse. And it is not enough for a writer to be mentally strong. Nor is cerebral fitness sufficient. Their minds must be in top condition, to write the tales which will be told for ages to come.