Cool Down and Refuel
Once you have crossed the finish line, don't stop abruptly! It is very important do some light jogging or easy walking for 10-15 minutes after you finish. This helps keep the blood from pooling in the lower extremities while helping the body remove the built up lactic acid within the muscles. And don't forget to do more thorough set of stretches while your body is still warm. Be sure to stretch the hamstrings (back of thighs), quadriceps (front of thighs), calves and upper body.
Once your cool down is complete, you need to rehydrate. You may also want to change into the dry clothing you brought in your bag. As your body's core temperature begins to drop, wearing damp, sweaty clothes can cause chills.
Drink plenty of water now and for the rest of the day. The best determination of your hydration level is by monitoring the color of your urine. The desired color is a pale like lemonade; a darker color means you may be dehydrated and lighter means you may be over-hydrated (and need to drink a sports drink such as Gatorade or PowerAde, both of which contain electrolytes to allow for proper sodium and potassium replacement).
You also need to make sure you get in a nice recovery snack as soon as possible after your race. Consuming a post-run snack containing carbs and protein will aid in muscle recovery as well as replenishing the glycogen stores within the body. Something as simple as a banana and peanut butter or low-fat chocolate milk will suffice.
Be sure to enjoy the festivities after the race is over. And be proud of yourself—you did it! HAPPY RUNNING!