I haven't done this event, but after thousands of miles of bad road on road bikes, here's my advice: Don't overthink this. You don't need a dedicated "gravel racer," and you don't need to spend the winter trying to convert an old mountain bike into one.
Fat tires, probably 25-28 mm, as long as they clear your frame with something extra for dirt buildup, are obvious. You'll be riding on pavement so cyclocross tires will be too slow, too squirrelly around corners, and probably too fat for your frame. Latex tubes, for their extra suppleness and puncture resistance, might be a good idea. Unless you're planning to do a lot of walking or hopping on and off the bike, your standard pedals and shoes will be fine. Shoe covers will keep your feet drier, or at least warmer. Two old racers' tricks worth considering are wearing two pairs of shorts and double- or triple-wrapping the handlebar. I recommend wearing a wool jersey or undershirt.
Make sure the bike is in an excellent state of tune--clean, well lubed, perfectly adjusted, with no worn out or soon to be worn out parts.
