It's kind of hard to be sure, really. Muscle can come back with exercise no matter how long the dog has been down, but exercise tolerance is affected by other things such as the heart, joints, etc., and like you say, she is a senior citizen. I think I would plan on just letting her do the minimum at first and try to observe how she seems to be feeling. One walk around the fence line of the back yard may be enough to tire her out when you start, you'll have to see. If that's the case, it can kind of make the owner think, "Gee, is it really worth putting the dog in the wheelchair for a 5-minute walk?" But yes, it absolutely is! It is great for the dog's mental health to be standing up in a normal position, even if it's just for 10 or 15 minutes standing in one place. It also is good for the circulation for the dog not to be lying on her hips and side and elbows, but to be standing so the blood can flow.
When my senior dog needed a quad cart (4 wheels), he was big and it was an effort to get him in, but I have this one photo of him just standing on the porch in his cart, not walking, just breathing the fresh air with a beautiful look on his face, and yes, it is worth it. I think the most happiness he had in those last months was being upright in his cart. He couldn't really walk, but he could stand tall again.
You'll have to see with your dog, they sometimes take to it right away and sometimes have to adjust. They didn't have Walkin' Wheels in the large size yet when my big dog needed his cart, so I never got to try it, but I know if you have any questions they will answer them for you, and you can send in photos of your dog to the company and they will check the fit and make suggestions.
I'm attaching a photo of my dog, and you'll see the happy look on his face. If you decide your dog is getting weak in front, too, because of age, there are attachments available for front support, you'll have to see how she does with 2 wheels.