It sounds like you are astute about your dog's physical condition. The goal is to have enough activity to stay fit and slowly strengthen, but not stress weaker parts of her body. There are definitely benefits to staying fit and developing good muscle tone. Muscle strength will help to alleviate the strain on spondylitic and arthritic joints. Swimming and hydrotherapy have lately become very highly rated for supporting muscle development and improving range of motion.
Your labrador may have days when she is just not feeling up for quite so much activity; pay attention to what she is telling you as you've been doing and adjust.
Basset hounds were not designed for long distance runs. As long as you respect that about the basset and again, pay attention to what the dog is telling you about her tolerance, then the activity will contribute to EVERYONE's health and wellbeing, including your own. BTW, there are very convincing studies in dogs showing that dogs lean and fit dogs live longer (on average) and age more gracefully than overweight dogs.
Basically, as I read your posting, it sounds like you are very observant, attuned to your dogs, and have a good amount of common sense. Carry on as you have been and your dogs should do fine.
Shannon Nakaya, DVM
Veterinarian/Author
www.kindredspiritkindredcare.com