Any dog with heart will always be your best dog. After running for three - four hours, I have to stop hunting and pull my dog away (she's only six years old). Later in the season, where I hunt, everything is under water. Its like hunting in cattails. Bird hunters have to compete with duck hunters as there are several duck blinds in the area. One big marsh. The dogs have to hop above the tall grass & water. The water can come up to waist high in some places to mid calf in others.
My dog will pull up lame if hunting about four hours - favoring her hind leg. Everyone usually quits after two hours. Anything more is hard on their dogs. Even with aspirin and glucosamine, it still takes the rest of day to recover. She'll will run herself to the ground if I let her.