Do you have any advice for a soon-to-be dog owner?

I haven’t had a dog since I was a teen (in other words, my parents had the dog). I’m planning to get a large dog, and will live in a city apartment no judging, please. I work at home, and have great access to nature and parks. I expect to walk the dog a couple of times per day.

For those who have been there before, and are perhaps there now, what surprises await me, good and bad?