sacred, that is a nice thing you have done for the old lady and her cat, very kind. Please keep her in at night as advised by the rspca because that is the time when they are most likely to come to harm, plus it's cruel in the winter. As she is your first cat, can I remind you to give her flea treatment or you will have a house infested with the little critters. I can recommend Advocate or Stronghold from the vet, don't waste your money on Frontline to which in lots of cases fleas have become immune.
I hope you and your cat have many happy years together, they are fascinating clever creatures. And if you have a problem you will always get good advice here. Enjoy. x