I am the owner of an 11 1/2 year old Great Dane and I would strongly encourage you to visit
www.greatdanelady.com. She has a complete holistic approach to caring for your new baby. First and foremost, you are going to need to feed anywhere from 3-4 cups of dry food twice a day. Some senior danes don't handle higher protein well and with that, you should look for a lower protein food. Look for foods under 25% but don't go too low as that can decrease muscle mass...which she will definitely need. Danes are much different than many breeds they age much quicker and can take more care to keep them going once they hit their older years. I just started mine on a new regimine and am anxious to see how it works. He is the oldest dane I have ever had and I am so blessed to have him still with me and fairly healthy!!! :-)
Good luck to you and congratulations on becoming mommy to one of the most awesome breeds ever!!! I commend you for taking her on and no doubt she will give you incredible joy!!!