@Gerd - This is just my gut feeling - In your examples, if I used "long ago," I did'nt know her personally, since she lived way before my time. To my mind, "a long time ago" would signify that I had in fact known her.

In my opinion Gerd, you could get by with both in spoken english. AE and BE are very lenient (or tolerant) when it comes to spoken English. But, when it comes to writing, I would use 'a long time ago.' It sounds better and looks better on paper.

For AE I agree with Carly. The phrase "long ago and far away" indicates a time before I was born and a place a long distance from where I have ever lived. "Long ago" also sounds (to me) just a bit old-fashioned, almost as if I were introducing a story I had heard. "A long time ago" can also introduce the above ideas, but also carries the idea of "within my lifetime", which - to me - "long ago" doesn't bear. But as Chichis noted, both AE and BE are very forgiving when it comes to speech acts.