Question

how does king use figurative language to express the idea that, though the journey is long and difficult, the civil rights movement will eventually be victorious

Answer 1

He refers to the "tortuous road" and "a super highway of justice." APEX.

Answer 2

King’s exquisitely powerful use of metaphors and his masterful employment of figurative speech, were the centerpiece of his rhetoric. He'd often compare two or more, sometimes vastly different things that were linked at least by one attribute to produce an even greater effect on the listener.

One famous example I would note:

His reference to the Emancipation Proclamation as “a great beacon of hope” in relation to the phrase “a joyous daybreak”. Here, he was effectively transmiting the point that as a beacon or  daybreak can illuminate hope, the Emancipation Proclamation would bring enlightenment and joy to all suffering slaves.

To address this question specifically he's comparing the journey to both a "tortuous road" and "a super highway of justice", at the same time. By doing so, King makes us acknowledge the hardships to come while immediately turning our focus up to the heights of the reward in the form of justice.

Hope this helps!