The part of the tooth that requires a root canal filling is one of those four dangly bits below. Just looking at it tells us how tricky it must be to do. I have had a few and only one was a wrong un. Blood weeped through and the pain from the pressure build up was phenomenal.
They take a long time and usually 2 visits to the dentist - I was in the chair for nearly two hours at the first appointment at the Endodontist then an hour and half for the second (luckily he had a very comfortable chair!) .... the one my dentist did was much shorter (probably 3/4 of an hour each appointment). The injection they give you makes it completely pain-free, but a bit uncomfortable having your mouth open for so long. It's not something you enjoy having, but to be able to eat and drink pain free made it worthwhile - the only other option would be to have tooth removed, and I really wasn't ready for that as it would've been visible when I smiled and if it avoided having a false tooth then I thought it was worth it.