Translation:I was reading your letters all day.
I will wait for natives in the team but I will note that "all day (long)" is definitely much more common https://books.google.com/ngrams/graph?content=running+the+whole+day%2Crunning+all+day%2C+running+the+entire+day
If I may ask, is there something in the Czech sentences of this lesson that seems to make this more progressive (I was doing VS I did VS to jsem dělal)? My English intuition is to say both translations are correct, since my understanding is Czech doesn't have a progressive, only past imperfective and past perfective. Am I wrong in thinking the Czech past imperfective can be either the English simple past or past progessive?