And if I say "I was doing exercises during the whole morning?"(isn't it correct?)(when I express that the action was being continused (continuous) in the certain period of time)
I don't agree you with first and second sentences,because your sentence means that the action is still continuoused (continuous) and I want to sat that I was doing it in the sertain (certain) period of time in the past)
Yes you can say that too: I was doing exercises during the whole morning.
Your own sentence in post #1 is correct too:
I was doing exercises for two hours.
or
I was exercising for two hours.
1. I have been doing exercises for two hours.
2. I have been exercising for two hours.
3. I did exercises for two hours.
4. I exercised for two hours.
4. I was doing exercises for two hours.
5. I was exercising for two hours.
6. I was doing exercises during the whole morning.