How to increase a german shepherd's lifespan?

I have a german shepherd who is about 12 years old. She seems pretty healthy except she is a little overweight. But I had a dream last night that bought me to tears. She had passed away. :( Of course it was just a dream but it bought me to a great realization that she is very old! She eats proper foods but it fed unnecessary treats about every other day. Also there is another dog in the house who seems to get more attention than her and she is really angry with him. But they did have puppies together about a year ago. And yes I mentioned that she was overweight. And I read somewhere that if dogs work out and run around outside that it increases their lifespan. So if you could recommend some ways I could get her in shape that would be great. But she is old and has leg pains…So it’s kind of hard for her. Please I beg of you, no negative answers. If all your going to say to me is “She’s not going to make it” or something like that please! Please! Don’t answer. It’ll just make me feel worse