The adverbial phrase "(for) a long time" just doesn't seem to go with the past continuous/progressive tense.
I can't imagine why I think that.
In fact, I think any adverbial phrase that specifies a period of time seems odd (just a little) with the past progressive.
The second of each of the following pairs seems better to me:
The children were sleeping for four hours. The children slept for four hours.
It was raining for three hours. It rained for three hours.
I was sitting in the study all day. I sat in the study all day.
I believe the adverbial puts the verb into a sort of 'event'
mode. It makes us want to envision what happened in the specified period
as an event (simple past), not as an activity (past progressive).
If we want the 'activity' interpretation, we can get that by interrupting with a "when" clause.
I was sitting in the study when suddenly a meteorite crashed through the ceiling.
But without this sort of contrast, the progressive just doesn't work as well.
CJ