This is the 501st most frequent Ukrainian word.
довго
"Довго" means "for a long time."
Here, 'довго' is used as an adverb describing the duration of the action of reading.
Він довго читав книгу.
He read the book for a long time.
In this sentence, 'довгою' is an adjective form describing the noun 'річка', indicating its length.
Ця річка є довгою.
This river is long.
The word 'довго' here is employed as an adverb emphasizing the extended time of the discussion action.