Semicolons, used that way (yes, properly) combine two very closely related, but otherwise complete sentences. In other words, "Paul knew he was lucky to have survived the gatecrash" stands alone as a sentence as does "Everyone told him he was lucky". According to Strunk and White, it's 'permissible' but not, as you point out, the best solution always, especially for an opener. It is, however, sometimes better than using "and" instead. A stylistic choice. (Another use for semicolons that is really lousy in most fiction prose, is in a list of phrases following a colon: like this phrase; another phrase, one with a comma this time; alternative uses for phrases, like this; and you get the idea. The preceding is, in my opinion, to be avoided with a passion.)
Now my entry:
It was going to take forever, Miles thought, to get a divorce from his wife, the windmill.