I am a native Russian speaker. There is a Past Continuous as well as a Past Simple tense in Russian, does that mean that my brain understands how the tenses work in English too? (English is not my native language, but I speak it somewhat fluently).
In general the meanings of a word or a construction in one language do not map directly onto the meanings of a word or construction in another language.
Furthermore, where there are different options available to speakers of one language in a particular context, there may not be a corresponding range of options in the other, and even if there is they often do not correspond exactly.
The "past continuous" in English generally has an imperfective sense, but the precise meaning can vary. The "imperfective past" in Russian generally has an imperfective sense, but the precise meaning can vary. Often the two will coincide, but their ranges will not usually overlap precisely - especially considering that in Russian there is ony the past perfective to contrast it with.
Or are you speaking of a participial form like я был де́лающий? I was never taught such a form, so I don't think it can be very common. Its meaning cannot therefore coincide with "I was reading", since part of the meaning of a phrase such as that inheres in the choice of that form against other possibilities.