I think you're okay - according to the chart below, 5 hours is about the cooking time you need for a 20 pounder - unstuffed, 4 1/2 to 5 hours, and stuffed, 4 3/4 to 5 1/2 hours.
Do you have a thermometer? Check the doneness with that, if you have one; if not, 'wiggle' the thigh and drumstick; if they're loose, - usually - the bird is done.
I'd watch the breast carefully to make sure it doesn't get too brown or dry; cover with some foil if it looks that way.
Do you have a platter large enough to hold that big fella?
Have a wonderful holiday!!