Start putting a little extra salt on your food the next couple of days.
Hydrate more than usual leading up to the race, starting a couple of days ahead.
Get a great night sleep two nights before because the night prior may be hit or miss depending on pre-race nerves.
Mentally prepare. Think about all your training and how you've conditioned your body. Know and believe that you can accomplish this goal.
Have fun! This is your first marathon. Enjoy the experience.
Short Story: My best friend did her first marathon years ago and it was about 40 degrees as well. At the start line, she said she felt the first drop of rain. And then it continued and got heavier as the day went on. In fact, it rained throughout the marathon and there were puddles everywhere with runners bitching and complaining.
My friend, however, was just so truly excited about running her first marathon that the rain didn't bother her. She hopped through and around puddles all day cheerfully saying to herself, "I'm running my first marathon! How cool is that!"
In other words, she focused on the positives and had an amazing experience.
Hope you do the same!