Fenders! First time I did a trip, I ended up buying them along the way: Big help with road grit.
I also had a fleece which I had put in a trashbag to keep it dry for the end of the day, that was a godsend. I ended up getting some knee warmers on the trip too, because my legs had gotten cold. A windbreaker at a minimum, and a breathable raincoat if your trip is long enough that you want to take it with you (some are heavy).
Riding in the rain = wet (either from sweat or rain)
Wet = cold
Riding cold = sucks
Minimizing the amount of riding in the rain you do will minimize your misery. Also, your body burns a lot more calories to stay warm, so remember to keep yourself fueled. Cold rain will constantly be stealing heat from any exposed skin, so do your best to cover up.