Thank you for the replies, everyone. Villain had 74o (bizarre, and I guess serves me right for not paying closer attention to how he was playing - but, whether it was 74o or AA I had to play the hand knowing I was behind, so it doesn't really matter) and I missed on the river. I'll try to post more hands like this in the future as I work on this aspect of my game, because I'm not going to get very far if I can't be aggressive without a made hand.