In the received Dao De Jing (Wang Bi recension), chapter 64 stacks three images, a tree that grows from a sprout, a nine-story tower that rises from baskets of earth, and a journey of a thousand li that begins under one's feet, then immediately warns: those who act on things ruin them, those who grasp things lose them. The Mawangdui silk manuscripts, sealed in 168 BCE and unearthed in 1973, carry the same passage with variant wording, confirming the line is early, not a modern embellishment.
The English form 'a journey of a thousand miles begins with a single step' circulates as a nudge to just start, popularized through twentieth-century quotation dictionaries and motivational publishing. It amputates the sentence from its chapter, inverts the warning into encouragement, and quietly upgrades li to miles.
Read inside chapter 64, the line is one clause in an argument for wu wei: the sage desires desirelessness and treats the end with the same care as the beginning, because people usually fail on the verge of completion. The thousand li are not about courage to begin; they are about refusing to grip the finish line.
If the original sentence is a caution against forcing outcomes, what else has the poster version talked you into gripping?