The poem is one long, unpunctuated command, and it keeps repeating the same word: walk. The speaker tells us to move steadily, not to run, even when something terrible is near. The odd comparisons make this feel urgent. Walking "as if the devil had commanded" it suggests a kind of grim obedience, as though the pace is forced on us. Then comes the strangest image, walking "like the prophecy of a horse when disaster comes." A horse can't speak, so its prophecy would be something felt in its body, in the way it moves.
The speaker doesn't say what the disaster is, and that vagueness adds to the tension. The tone seems calm but firm, almost like a quiet order given in an emergency. Walking becomes an act of control and dignity when panic would be easier. The lack of punctuation makes the lines roll forward without a pause, much like steady footsteps, and the final "walk" lands like one more step.