Good luck! Several years ago I could have done that along with you. Right now it would be too much.
You did not ask, but I need to tell you they sell special socks for runners. They are made from a special blend of cotton and other fibers and they kept me from getting blisters. See if you can get a pair of these -- they made an amazing difference over all cotton socks. I am sure you already have comfortable shoes picked out. Have fun.
I just saw that you are not walking alone. Is your dog used to long walks? Will you carry a little bowl and water bottle for them to get drinks along the way? Maybe even have someone handy in case the dog needs to drop out of the walk?
My dogs are little and would not last that long. Good luck to the dog too!