The orange pith is quite rich in pectin, and so are the pips! The skin and pith itself takes quite a few hours of cooking to tenderize them enough for eating. What I tend to do is to soak the oranges in water for at least 24hours prior to preparation and cooking. This assists in tenderizing the flesh, it also starts to release the pectin from the oranges which is so crucial for the setting.
Don't forget the pips! you will see that when they have soaked in water overnight that there will be a pectin jelly surrounding them as the pectin starts to release. There are so many methods of making marmalade but I prefer not to waste a single bit.
I usually cut off the rind with a knife or a peeler and chop these into whole lengths which will add to a good texture and shape. I separate out the pips and put them into a muslin bag during cooking, and I chop up the orange flesh too. I have started liquidizing the rest of the pith and adding that to the cooking mixture. Boil all this liquid for several hours with some added lemon juice. Acid helps to release the pectin into the liquid. When this has all boiled down for several hours this is when I remove the bag with the pips then add the sugar.
I have never found any published cooking times at all helpful! It might say boil it for 10 minutes but it can take hours depending on how much pectin you have to make a good set. Personally I find that pouring some liquid onto a frozen plate and allowing it to set for 10 minutes is the best guide to when it is cooked. 2 pounds of Seville oranges, 4 pints water, 2 pounds sugar, 2 lemons.Can anyone tell me if Seville oranges are allowed, grown or used in the USA? All the marmalade I have ever tasted over there is always made with sweet oranges.