Marathon minus 6

Ok so this is my second marathon, so why am I so nervous? I've checked the forecast, http://uk.weather.com/weather/dailyDetails-Newcastle-upon-Tyne-UKXX1695?dayNum=6 Wet and cold. so now I need to plan what to wear that will be comfortable to run in for 26 miles (5 hours) . Also struggling to get hydration right, the more I drink, the more I pee. I ran 5 miles tonight at a comfortable pace in the trainers that I plan to wear on Sunday. I've downloaded the driving directions to the car park, 500 mtrs from the start.