adverb
for a long time; since long ago; from childhood
Describes something that has been the case or known for a long time, often from early on. Can imply a deep-seated or long-held state.
彼の才能はつとに知られている。
His talent has been known for a long time.
つとに健康の大切さを説いてきた。
I have long been advocating the importance of health.
adverb
bright and early; early in the morning
Used to emphasize doing something very early in the morning, often with a sense of diligence or eagerness. Somewhat literary.
つとに起きて散歩に出かけた。
I got up bright and early and went for a walk.
早く is the common word for 'early' and is neutral in register. つとに is more literary and often implies a long-standing state or an especially early time.
昔から means 'since old times' and is more concrete about a distant past. つとに can refer to a long time up to the present without specifying a starting point.
Derived from classical Japanese. The kanji 夙 originally meant 'early morning' or 'long-standing', but the word is now almost exclusively written in kana.