Like the experience of warfare, the endurance of grave or terminal illness involves long periods of tedium and anxiety, punctuated by briefer interludes of stark terror and pain.
The post-presidency, as Jimmy Carter and Bill Clinton have proved, is a win-win. Money, Nobels, the ability to leverage your global celebrity for any cause or hobbyhorse you wish, plus freedom to grab the mike whenever the urge takes you without any terminal repercussions.
Write as if you were dying. At the same time, assume you write for an audience consisting solely of terminal patients. That is, after all, the case. What would you begin writing if you knew you would die soon? What could you say to a dying person that would not enrage by its triviality?