Anterior teeth have a single canal and #24 is no exception. If the tooth is not abscessed and hurting now, there should be no pain involved. The injection is more uncomfortable than the whole procedure since it will be given at the apex of the tooth to anesthetize only this one tooth. The procedure is probably scheduled for an hour but will only take a few minutes to actually open, clean out the canal, then enlarge it, irrigate or flush the canal out and disinfect it, then dry and fill the canal. The anterior can be done rather quickly due to the ease of reaching them and only having one canal to work with. It's always a good idea to get your pain medication filled and take one while you are still numb so you can allow the feeling to return slowly and comfortably. Most patients are fine with the anterior teeth but do play it safe the first day and night. Almost everyone have some discomfort the first night after the tooth is opened, this is normal. Be sure to take your prescribed pain medication along with an Ibuprofen, usually at a 600mg dose every 4-6 hours, do both of these that evening, even if you don't think you will need it just so you can rest comfortably. By tomorrow you can eliminate the pain medication if you feel comfortable.
Additional information: That is one reason why we like to place a patient on an antibiotic therapy, to reduce the amount or strength of an infection so that the anesthetic can be administered effectively. An infection will work by blocking or neutralizing any anesthetic keeping it from infiltrating through the tissue, making for a very uncomfortable, if not painful, root canal procedure. This is the reason most people have heard that root canals are unbearable or horrific procedures; it is because someone they know or a friend of a friend, had a really bad abscess requiring immediate attention and the patient’s health was more important than that of waiting on an antibiotic therapy to be effective. In cases like these, the anesthetic is worthless and the tooth must be first opened and then an anesthetic is placed directly into the canal where it will anesthetize the nerve and tooth enough that we may proceed with the treatment. Consider your self lucky to have been on antibiotics first, in that respect anyway.
Don't let this sudden loss of pain make you think the tooth healed its self, that doesn‘t happen. It may do fine for a few days or even weeks, but it will flare up again and usually it’s a much worse pain than before. I tell patients it’s like a volcano, building up the pressure until it reaches the lava stage and spews, the second time around is the lava stage for the tooth. Get the tooth taken care of as planned. I'm sure you will do just fine.
Additional information: Let us know how your procedure went yesterday, I always enjoy reading the feed back from patients who have had a procedure that’s considered to be a painful experience, especially when the procedure went really well.