If you don't exercise your dog properly and according to its needs, you'll both be unhappy. If you don't have time for the dog, it's best for both of you to wait to get a dog until you do have time. If you think you want a puppy, but aren't home most of the day, think again. Puppies need even more attention than adults. If you can't be home every 3 hours for an 8 week old puppy, you shouldn't have the puppy for this reason, if no other.
Every dog needs training.
Some people train their dogs to obedience titles, others just want a well-behaved dog. Training helps your dog understand what it can expect from you, and it helps you understand what you can expect from your dog. Your dog will love the time you spend with it, you'll love how your dog behaves, and you'll both be a lot happier. Unless your dog is VERY well trained and obedient, your dog should probably be on- leash or in a completely fenced area at all times. Many labs love to explore and roam, and unless you have trained your dog very well, it should be on a leash.