One benefit to getting an older do is that they are through their "puppy phase". Until my dog was about a year and a half old, he ripped things up, got into stuff he wasn't supposed to and was honestly more bother than fun. He's older now and much more laid back.
As far as walks and stuff, my dog lives on a farm, so he gets to run around as much as he wants. Sometimes he goes off hunting or whatever for a day or so and we don't see him at all. He eventually comes back, usually with something dead.
That said, I don't recommend getting a big dog if you don't plan on giving him enough exercise. Even smaller dogs need plenty of exercise. If you don't have a fenced-in yard or don't have a park you can take him to regularly, get a tiny dog.
