Hi,just adding a few ideas although a lot of useful info has already been offered.
We do not know where you are departing from and your arrival country but if its more than a 3 hour flight maybe consider a stop enroute say 2-3 hours to make use of the airport welfare facilities.Not possible on over the pond routes thou.
Maybe consider business class as often fewer wanting to use the facilities,this has helped me in the past.
I personally would not inform the airline before travel but when off the ground (when safe) have a chat with the senior cabin crew and explain you will need extra time in the wc,just mentioning the extra time as recently cabin crew had to open the wc due to my seat buddy falling a sleep in there,something to do with all the gin & tonics they gave him.
Have you asked your doctor for advice,some doctors will accept questions by post or email,so you do not have to visit their surgery.
Good luck with your travels.