Learn English – What’s the meaning of hit play

Two idioms here:

It takes time means "time is required". It takes a long time means "a long time is required".

Forever is a colloquial exaggeration, understood as "a very long time".

So which takes forever means "which requires a very long time".

The break-in or mechanical run-in is a standard procedure for a brand new engine, like those on the Titanic.

I think (and someone more familiar could confirm) that "run in" is the British term. It means the same thing as the Americanism "break in". To break something in is to take something brand new and use it in such a way that it reaches a higher capacity for performance. You might break in a new pair of leather boots so they fit you better and don't cause blisters when you walk, or the engines on a brand new ship.

I found "run in" in the Oxford dictionaries, confirming my theory that it's a Britishism used almost exactly the same way as the Americanism "break in" with respect to engines and mechanical things. The Brits also use "break in" the way Americans do for other (non-mechanical) things.