After a marathon it is time to take a well-deserved break. You will not make any gains by resuming running straight away. In fact you may risk an injury, your muscles and tendons have reduced resiliency and you probably have a series of micro injuries. Your immune system is also down, and this creates an opportunity to get sick.
The next couple of weeks make sure to eat protein, which helps build your muscles back up, carbohydrates, which help replenish depleted glycogen stores and drink plenty of water. Making sure to eat nutrient dense foods will help you in your recovery
There isn’t a calculator that tells you exactly when to start running again. I would suggest waiting a couple of weeks, which will help you recover properly. Listen to your body, if you have lingering soreness wait a little longer. If some part of your body is injured or nagging, go see a doctor to get it checked out.
Swimming, cycling and walking are great options for cross training. They will help get blood flow through to your muscles, which will help repair muscles. The key to this is to make sure you go easily.
Make sure to keep stretching, and if you can, get a massage. Recovery time depends on many factors, diet, sleep, general health, your overall fitness and a very large variety of other factors. Each marathon is different so don’t base your recovery time from your last marathon. Keep flexible yourself.
Once you do start running again, keep it easy and short. Slowly add miles and intensity back in. There’s plenty of time to start training for your next race. Allow yourself to fully recover and you’ll be in a great position to optimize your training. Congratulations on your marathon!