I am a dog walker who regularly runs and hikes with dogs. I would say a lab mix from a shelter would be the best fit for you. I work with a few smaller lab mixes (50ish pounds) and they are super athletic and well suited for hot or cold weather.

I have a german shepherd/basenji mix myself and she is very athletic and smart but any dog bread for hearding or hunting is gonna take longer to learn good off leash manners especially in the forest where there's so much prey.

I would avoid breeders because even the "good" ones are more likely to give you a slightly inbred dog who will have more health problems. A mutt with a more diverse gene pool will be much less likely to have joint/muscle problems.