I agree with you, that as long as a dog isn't suffering, and is just slowing down because of age, then life is worth living. 15 is very old for a dog and even going around the block at that age is quite a feat. I wonder if that's pushing her too much for her age. Exercise is good, but she's the equivalent of about 100 years old. I'm used to bigger dogs that don't live that long though. Smaller dogs live longer.
One thing that I didn't know about until I was in a situation that I found out, was that putting an animal down isn't pain-free for them. It's not the needle prick that I'm referring to, the shot is known to cause pain. I didn't know that and I felt terrible that the last thing my cat did was a yowl of pain when it was injected.
As my animals get older I dread losing them. I worry so much about doing the right thing for them, and how to know what the right thing is. Weighing all the variables. Ideally they live a full and happy life and just pass peacefully in their sleep when their body gives out.
Hang in there with her. She seems to be doing well for her age. Although I'd maybe shorten her walk some, but I'm not seeing her and I don't know her so maybe you're doing the right thing.
Word of advice...She may have days, months, a year, or even more to live, if her luck continues. But, make every day count