Oh, I think he was going to confront Clark (I have no idea why it took him all day, but I also don't know why these people are always driving from Smallville to Metropolis or vice versa to tell each other stuff they could tell each other on the *cell phones we know they all have*, either). And I think he believed there was a possibility Clark would kill him (after all, he just knows he has powers; he may not realize they pretty much make him *complete* invulnerable), which is why he took the time to tell Lana that he loved her (another visual echo -- it's the same way he grabbed her face in Commencement, only her reaction is completely different from what it was in that episode).

I took the meeting on the roof of Luthor Corp as Lex doing his usual big, grand, romantic gesture. And it is romantic: "If you decide to be with me, then join me at the top of the tallest building in Metropolis and look out over all that we'll survey together one day" or something along those lines. Also, he knew he'd just dropped something huge on her and unlike some people, who think a marriage proposal is the way to gloss over all the pesky implications of learning news that big about one's paramour, Lex recognized that Lana needed time to process and decide what she wanted to do. Meta-textually, I took it as a screenwriters' shout-out to "An Affair to Remember", so it didn't seem weird or odd to me but rather, like I said, romantic (what was weird was that he didn't give her a specific time, but this *is* SV we're talking about).