Lessons in adopting second-hand dogs: What I wish I had known

In what feels like a different lifetime, but in reality was a mere two weeks ago, we adopted our black Labrador Ruaridh (pronounced "Rory" — it's Gaelic) from a rehoming centre. Specifically, from the Dogs Trust — a UK charity that takes in dogs from the stray pound and from owners who can't look after them anymore, and finds them new "forever homes."
Almost without exception, the adoption centre staff were kind, practical and helpful in supporting us to find our ideal dog. Ruaridh's an absolute dream, and we wouldn't be without him for the world. What I found, however, was that dog owning manuals almost exclusively talked about "choosing your puppy from a litter," and "bringing your new puppy home." They were of little help in preparing us to visit a hectic, deafening kennel, adopting an almost-fully-grown, bouncy adolescent who's almost as strong as me, and dealing with two years worth of ingrained good and not-so-good habits. Here's what I wish I'd known…