And across the line! Way to go!
Once you're done, recovery begins. Even if you struggled with your pre-race preparation and hydration, there are still a lot of things you can do to keep from hurting too much in the coming days.
The most immediate thing to do is to keep moving. You ran hard and now you want to drop to the grass and enjoy the sensation of stillness. But this is not how your muscles, including your heart, are built.
Active rest and easy walking allow your body to adjust to the change of pace and let your heart rate come down gently. Unheralded sports physiologist Sir Isaac Newton once said, "An object in motion stays in motion." The object he was talking about was your heart. A 10- to 15-minute cool down will allow your heart rate to drop in a controlled, safe manner.
After you have been moving for a while, you need to stretch. Stretch everything. You're going to want to stretch your calves, quads, and hamstrings. Those are the obvious muscles you just spent an hour activating. But they aren't the only muscles you used.
Your neck is probably a little tight because you started holding stress. Same with your traps and shoulders. Also stretch your feet.
Don't stop drinking. You want to continue to take in fluids. Many athletes finish a race at least somewhat dehydrated. Grab some of that free sports drink or water from a smiling volunteer and sip it down. Don't chug—cold water hitting a blood-light stomach will only make you feel sick. The goal is for your pee to look like lemonade, not apple juice.
Some races spring for on-site massage therapists. Take advantage of this gift. A post-race massage should consist of smooth, long, soothing strokes. The therapist shouldn't dig for knots; this isn't the time for that. Your muscles have been abused enough for one day.