Here's an explanation of the use of the Past Continuous in Advanced Longman English for Intermadiate Students. In #1 we can read that the Past Continuous can be used just to emphasize duration, e.g. It was raining all day. Can I use this tense just to emphasize duration in the following situations:
1.Yesteday's homework was difficult - I was doing it for three hours.
2.Yesteday's homework was difficult - I was doing it ___ three hours.
3.Yesteday's homework was difficult - I was doing it all day.
If I were a native speaker of English, I would never shut up. :-)
Yes, but it took me three hours sounds more natural to me.