Set pushes the turn on a very scary board.

[fish43]

>>My feeling is that a decent, thinking opponent is going to know I'm not drawing to an inside straight for $20 on the flop<<

...unless you have a pair and an inside straight.

What is he thinking you called with? You have LAG stats, so he could put you on T8, 86, 76, 98, T9, 65. I think the range of hands that the villian calls with is smaller. If he thinks you are bluffing, then he still has to have a hand to call. I think he folds most of the time.