Rabid dogs are afraid of water, so... Throw water on it, it'll run away, its better than having to get shots later.
Dogs have small lungs compared to humans and a swift kick in the chest can take the breath right out of them. It's not easy to do and I wouldn't recommend it unless forced to defend yourself. The other option is to shove your fist down it's throat. You'll require rabies shots later, but it beats being torn to shreds.
tell her, " i'm a grown up now, Mum. **** off"
push the old lady towards the mad dog. don't move till it has a leg or arm securely in its jaws. Then you can get away. Don't forget to call 911. This is important in case this happens again.