How much education do you need to understand this sentence?

"When he found himself immured in a dank and noisome dungeon, and knew that all the grim darkness of a medieval fortress lay between him and the outer world of sunshine and well-metalled high roads where he had lately been so happy, disporting himself as if he had bought up every road in England, he flung himself at full length on the floor, and shed bitter tears, and abandoned himself to dark despair."

According to the "Flesch–Kincaid Grade Level Formula", 30 years of education is required.
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Where did it come from?  The classic children's book, The Wind in the Willows.