Checking the turn with KK and calling the river??

[DanK]

Great 6-12 game at the local casino, all full of elephants except for one good tight player.
UTG1 flods, UTG+1 (tight player) raises, old guy who plays every hand calls, MP1 fold, I make it three with red kings, folded to habitual caller on the buttons who calls all 3 cold, BB calls, good player caps, old guy calls two again, and the rest call.
Flop is Six, Five, Two, with two diamonds. Tight player bets, old guy calls, button and BB get out, tight player makes it three, old guy calls two again, and I cap.
Turn is an eight of diamonds, giving me a fluch draw.
It's checked to me. Now I'm sure the good player has an overpair, as he would never raise from EP with a small pair, and I'm almost certain he knows I got an overpair too. But the old guy got to have something and I think he might have just hit his fush, or hes hanging on with ace six. I'm not sure if he's sophisticated enogh to check raise, but I still just ckeck. River is another six good player checks and old guy comes to life and bets. I make a call sure I'm up against trips now, and tight player mucks red queens face up. Old guy shows seven four off for the stright and I muck.
So how bad was the check on the turn and the call on the river. All comments appreciated.