Your pet food label should have some feeding guidelines according to your dog's weight. If you leave the food down all day you can either put that amount down all at once or divide it up and give it twice a day. They are just guidelines though. If your dog is very active then he may need more and if he is a couch potato he may need a LITTLE less.
Taking him out for at least one good brisk walk a day will help him to take off weight and strengthen his muscles. Muscle strength is important as he gets older and joints start to wear. Those muscles and ligaments help support those joints.
We play indoor fetch when the weather is bad and I can't take Gracie for a good walk. She needs to run off her energy or she gets a bit too mischievous!