I am on-line another 15 minutes tonight to assist you both
If it is cooler weather and he is having a slower time of recovery after an hour of exertion it can potentially be concerning. If it is progressive or not improving it would be ideal to have your veterinarian auscult his heart and lungs to ensure there are no irregularities that require management. There are muscle
enzyme issues that can occur but typically this is a Labrador problem not something we see in Golden's. If he is evaluated and has no issues whatsoever and it continues and/ or progresses then I would consider talking to your vet about testing him for one of the conditions such as the one below (again not the right breed but unfortunately they do not always fit into the mold of breed, age, sex).
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