15 minutes only would of killed me when 'Lo was young. I'd walk with her and D as long as she was still playful. Usually that was 1/2 to 3/4 mile (at around 12 weeks) but me and D, being used to going two miles at least, let her go as far as she'd like. She'd get several off leash runs through the day as well. And if she wanted to do something, she could in general. Only think I wouldn't do was let her sprint on hard roads for long.
And she was, and still is, small for the breed. She's 14 inches tall and 13 pounds, a larger dog should be able to do more. And she's fine and healthy at three years.