Yes, that is the meaning. Personally I would only use "long" like this with stative or continuous verbs. For example, "He had long disliked her" or "He had long been a supporter".
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Edit: Actually, it may be that "come to the conclusion" is intende
park sang joonThen I was wondering if you think "for" is implied rather than "ago."If "come to the conclusion" is intended to refer to a discrete event then "long ago" would be right.