During the first three to seven days after a marathon you are learning about delayed-onset muscle soreness. Delayed-onset muscle soreness is microscopic damage to the muscle fibers and surrounding tissues caused by the stress of a marathon and it is going to be painful as you can all attest. Often the most severe part comes 24 to 72 hours after the race.
Dealing with delayed-onset muscle soreness is crucial to proper recovery. Get a massage, go swimming, ride a bike, take a walk. But don't run until the soreness in your muscles subsides. Your muscles' resiliency to injury is at an all-time low, and your risk of injury is very high. These other forms of gentle exercise will pump blood to your muscles and help you recover more quickly. Some people will claim they feel fine after a marathon and will want run right away. Fight the urge to head right back out the door and allow your body the proper recovery it needs.
There is another reason we need to take time off. Eventually our warped judgment will lead us to train for another race. But for now, your mind needs a break from running. During the recess, indulge yourself in hobbies and activities that you may have steered clear of during training. Give your brain a rest from the discipline of training. After a week the soreness should be gone. If not, another massage should work wonders.
You are probably compulsive enough to resume training in a week or so. How should you re-build your mileage? Try a pre-marathon taper in reverse. In you first week back to training you should jog a few miles 3-4 days per week on a soft surface like a track or the inner loop at Sharon Woods, the pet trail at High Banks or the new Rocks and Roots Trails at Alum Creek. Maybe 12 - 18 miles total. During your second week run around 50 per cent of your usual weekly mileage and then the week after that 75 percent. We can argue what we mean by "usual" weekly. Your usual weekly mileage is not your peak mileage. Instead, use the average of your weekly mileage over the full duration of last season. For those of you who need to see this on paper click here!
At the end of your 75 percent week try running a half mile at your 10k race pace. About 25 seconds below half marathon race pace. If you can run fluidly without stiffness you can graduate to your normal mileage. If you are still stiff give yourself a week at 80 percent and then at the end of that week do the half mile test again. After 3 or 4 weeks of normal training - or seven to eight weeks post marathon, you should be completely recovered from the marathon and ready to consider racing. The first race after a marathon - like a 5k - is going to be a little slow. The marathon has made us very strong but we may have lost some leg speed. Don't worry - it will come back.
We have seen unfortunately more people than we can possibly count who do not take the time to recover from a marathon. Instead they jump into training right away or worse yet other races. For these people they often never fully recovery and the cycle of doing a marathon and then jumping into another drags on their performance and their bodies ability to recover. You have all worked too hard to not leverage this work, allow your body to recover, and come back stronger next season.