He should be in good muscle-tone, with no excess weight to hamper his, hopefully, flowing movement. Muscles should be rock-hard, not spongy and soft. When you place your hand on his ribcage, you should be able to feel his ribs easily without having to push down and feel around for them.
Proper conditioning follows a very fine line.
It is the correct balance between good nutrition, amount of food, and exercise. First the dog should be gotten into his proper weight. This will vary from dog to dog, depending on his height and density of bone. Measure his food out every day. Obviously, if he needs to diet, he should be fed a bit less until the correct weight is achieved.