Eat after root canal. I would suggest that you wait to eat hard or semi-soft things until after you anesthesia has worn off. I would also caution doing any heavy biting or chewing on the root canaled tooth until you put a permanent restoration on the tooth. Root canaled teeth tend to become brittle over time, and you could end up damaging your tooth beyond repair. Root canaled teeth can be sore up to 3 days afterwards.
Soon. Though it may be sore you shoud be able to soon depending on the material used for the filling after the root canal was done. Ask your dentist. Mine are usually either right away or after a couple hours, just nothing hard or crunchy for the day.