That sounds like a fine length for a walk for now. If your dog seems tired though, then shorten it.
Also, I have to say people often underestimate the stamina of small dogs. It does depend on the dog, but many small dogs, particularly terriers, can be little energizer bunnies. Our 10 lb Tyrion is a Maltese/Yorkie (with bad knees no less!) and sometimes I think he could run all day long chasing my Cattle Dog mix. What sort of dog do you have? That might be an indicator of how much energy/stamina he will have. I do think the biggest difference though is that while a small and large dog might both be able to do an hour long walk, the small dog might have trouble keeping up with the big one.
Also, I have to say people often underestimate the stamina of small dogs. It does depend on the dog, but many small dogs, particularly terriers, can be little energizer bunnies. Our 10 lb Tyrion is a Maltese/Yorkie (with bad knees no less!) and sometimes I think he could run all day long chasing my Cattle Dog mix. What sort of dog do you have? That might be an indicator of how much energy/stamina he will have. I do think the biggest difference though is that while a small and large dog might both be able to do an hour long walk, the small dog might have trouble keeping up with the big one.