The past perfect continuous and past continuous tenses are both used to talk about actions or situations that were ongoing in the past. The past continuous simply shows continuity, while the past perfect continuous emphasizes the duration of a past activity or state, mainly used to indicate how long a past action was happening.

12. Difference between past perfect continuous
and past continuous tenses
Past Perfect Continuous and Past Continuous can be used to talk
about actions or situations that were in progress at a certain point of
time in the past. While the past continuous merely shows continuity,
the past perfect continuous tense also puts an emphasis on the idea of
duration. However, it is mainly used to indicate the duration of a past
activity or state.