I don't think fitness is that important. Get out and so some short tours together, test out your equipment, attitude compatibility, that kind of thing, and get an idea how far you and your partner can go in a day, comfortably, fully loaded, while still having fun. Plan your vacation accordingly; don't plan on being in better physical condition than you are now. I'm sure you will improve your fitness, and that's great; but it's not the most important.