Dogs need time off lead incorporated into their daily walks.
A good run in the park with his chums is great for getting the heart going, getting blood and oxygen into the muscles, stretching out the tendons and ligaments, strengthening the bones and joints, allowing the skin and coat to breath and preventing future injuries, as well as longer term benefits like maintaining weight and the cardiovascular system.
The mental benefits are enormous too. A well-exercised dog is a happy dog, more focused in training, calmer at home and in new situations, and more confident with unfamiliar dogs or humans. Regular exercise also reduces the likelihood of depression and behavioural problems, like hyperactivity or aggression.
Keeping a dog on lead all the time is more likely to cause more problems, more bad behaviours, than giving it a run, allowing it to follow a scent, to pick up 'messages' from other dogs, to work on and improve recall and to tire itself out mentally as well as physically.