All seem slightly awkward to me, sorry. I don't see any difference in register, though-- they are all rather informal. If I were composing the idea, I think I would try It was long and boring , and leave it at that.
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English 1b3a. It wasn't so much boring as (it was) long.I'd use
b. It wasn't boring so much as long.
c. It wasn't so much that it was boring, more that it was long.