Dated adverb; in modern Japanese, 早く or 速く are far more common. Still encountered in older literature or fixed expressions.
He quickly left.
noun, noun which may take the genitive case particle 'no'
Dated noun/adjectival use; often appears in the phrase とうの昔 (とうのむかし) meaning 'long ago' or 'ages ago'. Modern equivalents include ずっと前 or とっくに.
That's a story that ended a long time ago.
long ago; already
since long ago; for a long time
Usually written in kana; the kanji form 疾う is dated and rarely used.
Dated kanji spelling; learners will almost always encounter the kana form.
The adverb とう is the historical ウ音便 form of とく, the adverbial form of the classical adjective とし, “swift; early.”