Rossetti is comparing the journey through life to death and to the existence after death as an uphill journey.
She starts by asking if the trip goes uphill "all the way" and is assured that it does so "to the very end" - throughout life, one is coming closer to the moment of one's death. She asks if the journey will "take the whole long day?" and receives the answer that it will, indeed, take from one's birth in the "morn" until the "night" of death.
She asks if there is a place of rest available when the night arrives and expresses concern that, in the darkness of the night, she might miss this refuge. The assurance is given that "you cannot miss inn."
She is also reassured that she will not be alone. Others "who have gone before" will be at this place of rest in the night nda will not prevent her from joining them. Finally, she is told that she will find "comfort" there after her travels up the hill of life, and that there is room in the inn for "all who seek" and "all who come."