The 40 mins would be an on-lead walk. Off -lead exercise in your own garden should be as much as the dog wants. On-lead they HAVE to walk with you, off lead they can stop & rest when their body says enough. This does not mean that you play, for e.g. fetch, in the garden to the point that your dog is exhausted, merely that running about under his own steam is fine as he will rest when tired.