Yes, good explanation, Simon.
Your condition won't stop you getting surgery, Jayne. Just think of all the crumbly old grannies that have been severely disabled with their bad knees for years! But it will make your recovery a whole lot easier and faster. As will the fact that your knees are skinny! We love skinny knees to operate on!
As for the time, a single knee takes about 2 hours including all the prep so you'd be excused for expecting a bilateral to take 4 hours. Well, sometimes a surgeon will get a colleague to come and work with him and they each do a knee at the same time. This is called a simultaeous BTKR. That will still take a bit longer than one as prepping two knees takes a bit longer. Plus there are certain points in the procedure where one surgeon will have to stop and allow the other one some extra space.
However, if the one surgeon does one knee and then the other, that can take about 4½ or even 5 hours as the team has to clear up and tidy between knees! Just a bit of housekeeping in order to keep things in order, y'know!