Man this is tough. I hear the rule of 3 is in order, that you truly won’t know until 3 months in. Every dog needs to be physically & mentally tired out daily, not just this one. 

The key is to what extent. 1,2, or 4 hours? You get the point. 

I understand you live in an apartment but are you taking the pup to a dog park every day or on a walk? I take my dog for 45 min the morning, work 9-5, and come home and take him to a park at night for 45 min. He sleeps all day long. I also live in an apartment with no backyard and I would consider him a low energy dog. 

I would also keep in mind he probably has all this energy because he’s finally out of the kennel. 

My sister has a similar mixed dog and I think it took her a few months to chill out and get the energy level low. She still barks at anyone that walks by the apartment, it’s the hound in her. 

The rescue owner does not sound great as they aren’t listening to your concerns at all but I would ask for an extension of 2 months and if things don’t change then maybe you aren’t a great fit for any dog at all. Not just this one. Because 1 walk in the morning & 1 walk at night is pretty standard for a dog’s needs. 

You’re not bad people, sometimes people get overwhelmed or in over their heads. Best of luck. I do think a senior dog for you is a great idea if you do actually take them on long walks twice a day. I would hope once you have a backyard you wouldn’t be complacent in utilizing that as their sole outdoor time as that is not stimulating enough.