Marathon, which is a real big race can obviously the reason after which you can get tired and need rest for some days. But these some days should be a short span of time. It is very important for an athlete to maintain his body fitness after an event again otherwise his body will breakdown totally which will be a huge and a real bad problem for him.
After the race of marathon, the highest priorities for your body in terms of recovery should be muscle repair, restocking muscle glycogen stores, and rehydrating. These are the real important factors just to maintain the body fitness after this big race. Try to get back to training as soon as possible so that your habit of running does not get loose.
After some days rest for like 2-3 days, try to take easy runs of 3 miles or 5 miles each day. Slowly after that you can take your step for a long run for 10 miles, in an easy way but not in a hurry. Running uphill and sprinting 4 miles easy at full speed which is good for your fitness. If you need, you can take a rest of one day again and again start warm up from the next day.
Once you return back to your normal training, you will find yourself just as fit as you were before and you can surely take up another race event for a challenge without any hassle. Stay fit and enjoy the life.