Not sure if this fits the context: "to be all over the map", e.g., "they were all over the map in their discussion about the details".

This is rather colloquial, appropriate for (spoken) business usage, but not for formal written use.

Could that be the equivalent?

Isn't that the equivalent for "von Pontius zu Pilatus", which is quite different in meaning?

Hi, Doris. Nice to have you back!
And, to begin with: Yes, you are right (of course):-).
What do you think of the new suggestion?

Morris happily admits that her book is self-indulgent. As she wanders aimlessly around the city streets so too her mind wanders. She goes off at a tangent exploring themes and preoccupations in her life.

@Reinhard: thanks - it's also nice to be back. I almost missed this forum. Almost...
But to get back to the burning issue of the little piece of wood and the little stick - I like your suggestion. You're definitely getting closer. But I'm still not entirely convinced about "talking nineteen to the dozen" since it doesn't put enough emphasis on "going off topic" which I think is implied in the German. Or isn't that the case?
Let me suggest "flying/going off at a tangent" (see example) even though I wouldn't necessarily translate this expression with "vom Hölzchen auf's Stöckchen kommen".
Or how about "to get carried away" - doesn't sound as "woody" as my previous suggestion and might just fit nicely in many contexts.
Other suggestions: blathering/rambling/rattling on - those, however, are more in line with talking nineteen to the dozen.

Talking nineteen to the dozen is talking a LOT and FAST, it doesn't necessarily mean going off the subject. Doris's "going off at a tangent" sounds good

I think this is what we would say in English when there is an unfocused discussion - basically, to take a long and unnecessary route to your destination!

Und hier die 'Rel. discussion' um einen verwandten Begriff:

related discussion: vom Hundertsten ins Tausendste kommen
. . .