This is the 111th most frequent Finnish word.
pitkään
"For a long time."
The word 'pitkään' is used as an adverb describing the prolonged duration of sitting and thinking.
Istuin pitkään ja mietin elämää.
I sat for a long time and pondered life.
In this sentence, 'pitkään' indicates the extended timeframe of the evening reaching into the night.
Ilta venyi pitkään yöhön.
The evening extended late into the night.
The word 'pitkään' here is an adverb indicating the duration of the waiting activity.