If it is possible for one word to summarize a man, then that word would be the advice he offered in Walden: “Simplify, simplify”[which is actually two words]. Thoreau was a self-consciously simple man who organized his life around basic truths. He listened to the inner voice of his conscience, a voice all men possess but few men follow. As he explained in Walden,
To be a philosopher is not merely to have subtle thoughts, nor even to found a school, but so to love wisdom as to live according to its dictates, a life of simplicity, independence, magnanimity and trust. It is to solve some of the problems of life, not only theoretically, but practically.
 loftiness of spirit enabling one to bear trouble calmly, to disdain meanness and pettiness, and to display a noble generosity
In coming to a definition of “living a peaceful life”, it occurs to me that key elements may be “simplicity, independence, magnanimity and trust“.