After years of constant socialising, he began to value the solitude of his early morning walks.

solitude 是名词，由形容词 sole（单独的）加上后缀 -itude（表示状态或性质）构成，意思是"独处、孤独的状态"。这个词强调一个人独自时的宁静状态，通常带有积极含义。