If the outside temperature is significantly different than you had anticipated, don't worry. You still have time to adapt. Better to know this now than to find out halfway through your race. Here's what you can do:
Know that you'll be less likely to want to drink, so make a note to stick with the proper hydration cycle. Don't make up for the morning temps with excessive hot beverages as they'll function like a diuretic, robbing your body of critical H2O. Instead, cover your extremities like your head and hands with some lightweight gear that can easily be stowed or tossed to a friend. Plan on being very warm to the race site (and afterwards), reducing your body's need to create warmth and waste valuable energy.
This can be the race killer; so be very careful here. Consider adding more fluids (sports drink) to your morning routine; as well as bumping up your hydration cycle from 15-minute to 10-minute intervals. Any calories you take in will require a bit more fluid than ususal, so be sure to eat close to the aid stations, and slow down as required to get in as much as you want.
More: 3 Ways to Run Through the Heat
You'll want some sunscreen and coverage, if possible, on your head / eyes. Avoid pouring water over yourself unless it's a last resort; holding ice in your hands is just as effective at cooling you off. And don't forget, wiping your face can make a difference too.
A big fear is to have serious head wind come race day; that's essentially like running uphill for the better part of your day. It can take a real physical and mental toll. If you find the winds in your face, don't focus on the pace numbers on your watch. Instead, focus on the effort you are running. Your 9-minute/mile effort into a headwind might only get you a 9:30 minute/mile pace, but that's preferable than having you run an 8:30 minute/mile effort to earn the 9-minute pace you want, only to implode long before the finish line.
More: 4 Cold-Weather Running Tips for Beginners
Conversely, don't "chase" a tailwind. It's tempting to run much faster when you know there's a wind at your back, but all that speed isn't exactly free. You are still running harder than you've trained, and the cost of that subtle change won't be evident until too late. Also, remember that without wind in your face you'll actually feel hotter than it is, so hydrate accordingly.
Don't be afraid to draft off of a similarly paced runner; simply slide in behind or next to him/her such that they stand between you and the wind. It's not so much about free speed as it is getting a break from the full effect of the wind.
More: 9 Tips for Running in Cold Weather