An adult rescue is an excellent option. I got Lucy at 2 1/2, she's 12 now and it's been great so far.
My thoughts, on a long healthy relationship.. .............. ......lots of exercise, top quality food - read lables carefully, and be very picky when you screen your vet. Do your homework, know what danes are prone to and apply the knowledge when you seek treatment for your dane. Example, weak hearts can be a common problem for Danes, if the vet recomends a medication that will stress the heart, be direct in you questions and maybe seek another opinion. I strongly believe Danes can have a long life with good care, and I've known a bunch of danes since I got Lucy that seem to prove that out.
On behavior (human and Canine), I'm afraid Lucy has been very bad. She is way too tolerant of peoples bad habits and I fear for some of the folks who've approached Lucy improperly, got away with it and will one day get schooled by a dog less accepting of human foibles. Somebodies gonna get bit, and it will be thier own damn fault, and they won't have a clue. Danes can be real marshmellows, be prepared to tell people when they're being idiots, it is for their safety and the dogs they meet in the future. Some states are pretty harsh about dog bites, and sometimes it's the persons own fault.
Last observation, Be prepared to be a public specticle. I'm not kidding, taking Lucy out to a public place is like walking down the street with a naked super model. Your Dane won't be much different. Answering the same script of questions can get tedious, never getting a moments peace at the beach gets old. Lucy is quite comfortable as a good will ambassador to all, you'll get to work that one out with your new friend for yourself.
Good Luck, try to love your new friend as much as he or she will love you, you can't but try any way
Gene