"You’re not ready to run another marathon until you’ve forgotten the last one."- Frank Shorter
Once you've completed your marathon, consider yourself injured. More than likely you are dehydrated, depleted, and inflamed. Your sodium levels are probably low, and your immune system is suppressed. Core body temperature is elevated and your legs are sore- usually very sore. Soreness is a sign that those muscle fibers have been severely traumatized and are damaged.
In order to repair properly, you need to eat, drink, and rest. Replace your electrolytes and start snacking as soon as possible. Dr. Tim Noakes cautions that a small number of slower runners who drank large quantities during the race might actually be over hydrated and should be careful not to drink too much in the period afterward.
If you sit for too long, you'll become very stiff, so I like to walk as much as possible in the first hour. Once fully re hydrated, over the counter anti- inflammatory should help reduce inflammation and speed recovery.
This article from sportsinjurybulletin.com discusses how antioxidant supplements like vitamins C and E can speed recovery of traumatized muscle tissue. Antioxidants may also protect you from post race illness.
Many marathons mistakenly believe that jogging the day following a marathon speeds recovery. You are better off resting until the soreness is gone before resuming light jogging. For most people, that takes 3-4 days. A very light massage may also help heal tissues, but basically you just need rest. If you must exercise, head to the nearest pool.
After 26.2 miles you are entitled to eat just about anything you want at that first meal. You certainly deserve it, and one incidence of over indulgence following that kind of effort means nothing. Ideally you should begin replacing healthy portions of carbohydrates and protein as soon as you can. Surprisingly, you may not have much of an appetite at first, but there is no doubt that you will be hungry within 2-3 hours.
The general rule of thumb for complete recovery is 1 day for every mile raced, so figure it takes a month to get everything back to normal after a marathon.