Past Perfect Continuous vs. Past Continuous

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.