I’ve been thinking about this. Maybe this of with old means that she was fed up with her job. I haven’t checked any dictionary or anything, but that’s a possibility. Maybe you have an archaic use of old in play there.
Of an evening may be related to other Germanic languages’ time expressions, such as des Tages, des Abends and strangely des Nachts (I say strange because des comes before neuter and masculine nouns in the genitive case, not before feminine nouns like Nacht, which would normally require der). Dutch also has ‘s avonds, ‘s middags and ‘s ochtends.
"Of an evening" is used quite commonly in N.W. England where I grew up.
Just did a quick Google on "too old of" and came up with 490 hits. Half of them straddle two sentences: I am too old. Of course….
Most of the rest seem to be a construction I am unfamiliar with: "Dos is too old of an OS", "Dallas looks like too old of a team to win" etc. In this construction I would simply scrub the "of". (Too old an OS, to old a team etc). Is this normal in the US, or regional? Perhaps I’m just out of touch.
Anyway, purely from the context of the original post I would say something along the lines of "rather too long in her job".
Now that you mention it, I grew up hearing that construction all the time, here in the southeastern US, i.e., This is much too old of a car to try to make such a long trip away from home.
However, the original quote still sounds quite odd:
I once knew a secretary rather too old of her job and quite depressed anyway.
Too tired of her job, maybe, or too sick of her job.
But too old of her job just sounds wrong.
I think it’s the possessive pronoun that just ruins the expression.
The secretary was very experienced in her job but, despite this fact, she was quite depressed.
Otherwise it sounds senseless.
If one uses ‘old in’ instead of ‘old of’, that leads to a misunderstanding: she was older than her colleagues.
If one uses ‘old for’ instead of ‘old of’, there’s a new misunderstanding: she has reached the age to be sacked!
