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I have been spending my MLK Jr. day googling these kinds of out of context quotes and trying to find their original source. Google books doesn't work - you have to sort through a LOT of misuse in "motivational" texts. This is FWIW is from his 1960 address at Spelman college. https://twitter.com/MHodler/status/1351176843748134913
*I'm not a King scholar I'm just a climate reporter.
In the speech, he begins by talking about Israelites path out of Egypt "they discovered that before they could get to the promised land they would have to go through a long and difficult wilderness. "
In the speech, he begins by talking about Israelites path out of Egypt "they discovered that before they could get to the promised land they would have to go through a long and difficult wilderness. "
He's using the Bible passage to make an analogy to the plight that Black people in the US are facing and encouraging them to move forward and saying there's temptation when moving away from slavery to get bogged down.
He's saying that the journey to literal freedom might be hard but the goal is to keep moving. But yeah, I guess you can use that for your workout motivation 
Full link (also the best guess is that specific sentence that swimming tweeted is a paraphrase of a passage from the Book of Isaiah) https://kinginstitute.stanford.edu/king-papers/documents/keep-moving-mountain-address-spelman-college-10-april-1960
