Going to have a root canal tomorrow. How bad does it hurts and for how long will I feel the pain? ?

It very much depends, but I have had a couple and they weren't so bad.  Depends on how well the root nerve is numbed before they dig it out. It is somewhat like having a cavity drilled without any local anesthetic (as was my experience in the 1960s).  The nerve will let you know if it is still feeling, and it is a sharp one.  Should not be any sharp response, but there could be.  If there is, it is fairly quick even if very unpleasant at the moment.

I don't really remember any major problems afterward.  Just the usual "my mouth was just worked on and is sore" for a few days. Still way better than the bad feelings I had from what made me get the root canal.

Perhaps the hardest thing to get used to is the lack of feeling from that tooth.  It feels weird at first.  You don't feel the tooth itself, but only the tooth interacting with the adjacent gums.