"Uphill" by Christina Rossetti is written in the form of a dialogue, with one speaker asking questions and the other speaker answering. Although it does not use quotation marks or identify the speakers by name, the pattern of question and answer is quite regular.
The poem is written in open quatrains, rhymed ABAB CDCD etc. The odd-numbered lines are questions and the even numbered lines are responses. The questioner is asking about a journey. The answerer says that it will take the whole day from morning until night.
The reason the answerer and the questioner assume that it will be dark when the questioner arrives at the inn is that the journey starts at daybreak and ends at night. The beginning of the journey is accomplished in daylight, but it ends after sunset. They assume that it will be dark at the end of the journey because it is dark at night after the sun has set.