did = felt It is another way of writing, "I've never felt such boredom as I felt watching 'The Lord of the Rings'."

"I did" and "watching..." are separate parts of the sentence. You could equally say "... as I did while watching "Lord of the Rings"". "Watching "Lord of the Rings"" serves a similar function as "看着LOTR" in Chinese, implying simultaneity. I'm not sure you could use this construction when translating this particular sentence into Chinese. I think you'd say something like 看LOTR以前，我没有感过那么无聊 (literally: Before watching Lord of the Rings, I'd never felt so bored). I can't figure out how to translate it in a way more similar to the English sentence you have there. Is it possible to say 我(别时)没有感我看着LOTR的无聊?

I think the confusion comes from thinking that "did watching" is a phrase. These words are actually separate parts of the sentence. "I've never felt such boredom as I did... (doing what?) ...watching 'The Lord of the Rings'." Or in other words: "What were you doing when you felt such boredom?" "I was watching 'The Lord of the Rings'."