Past Perfect Continuous vs. Past Continuous

Question

Every now and then I come up with a sentence, that eventually undermines my confidence as to whether it's correct or not - just a part of my OCD issues I guess.

Same thing happened yesterday and since then I've been dwelling on that, unable to find any examples that could prop up my thesis. The sentence I formulated was:

Do you expect me to believe that you were running for 20 minutes flat?

My concern is: did I get tenses right or perhaps I should've used "had been running"? Can I use "for" and "since" with past continuous while I'm talking past unrelated to present, or are those used solely with perfect tenses? As in:

They were fighting Nazis for three days.

Or

I was studying for six hours yesterday.

Or

I was working there since I graduated from school.

These things are pretty basic, but I tell you: OCD may really get on your nerves and make you doubt your own skills. Therefore I just want to make sure if these are correct.

Answer 1

Past perfect forms, continuous and non-continuous, are appropriate only if there is some implied or explicit reference to one action starting before a past time.

Do you really expect me to believe that you were running for twenty minutes flat yesterday?

Do you really expect me to believe that you had been running for twenty minutes flat when you passed me yesterday?

.

They were fighting Nazis for three days.
I was studying for six hours yesterday.
I was working there since I graduated from school.

The first two are fine; the third is more doubtful. We normally use since for a time period starting at one point in time and continuing up to another; the verb is normally present or past perfect. Thus we would say:

I have been working there since I graduated from school. - The time period extends up to the moment of speaking.

I had been working there since I graduated from school. The time period extends up to some past time already referred to.

However Michael Swan notes (Practical English Usage, 2005.522) "present and past tenses are also occasionally found". The third sentence therefore seems possible.