57! In my book, that's relatively young.
I don't think we should live life thinking about our demise. Of course, it makes sense to take reasonable precautions but, in my view, that shouldn't preclude having a pet - if you want a dog, get a dog.
I believe the Dog's Trust has some sort of scheme whereby they guarantee that, should something happen to you, your dog will be cared for. Possibly there are other charities which have similar schemes.
A friend of mine has three dogs (and a cat) and she is 84. I believe they keep her fit and healthy, both mentally and physically.
We had to have our dog put to sleep in July and I miss him very much. He was wonderful company and a reason to get out in the morning and evening.
If you are reasonably fit, I think it would do you the world of good.