AK in late position

[illini43]

Was this the right play?

PokerStars 0.25/0.50 Hold'em (9 handed) converter

Preflop: Hero is CO with K[img]/images/graemlins/club.gif[/img], A[img]/images/graemlins/heart.gif[/img].
<font color="#666666">1 fold</font>, UTG+1 calls, <font color="#CC3333">MP1 raises</font>, <font color="#666666">1 fold</font>, MP3 calls, Hero calls, <font color="#666666">1 fold</font>, SB calls, BB calls, UTG+1 calls.

Flop: (12 SB) 6[img]/images/graemlins/club.gif[/img], A[img]/images/graemlins/diamond.gif[/img], 5[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(6 players)</font>
SB checks, BB checks, UTG+1 checks, MP1 checks, MP3 checks, <font color="#CC3333">Hero bets</font>, SB folds, BB folds, UTG+1 folds, MP1 calls, MP3 calls.

Turn: (7.50 BB) T[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(3 players)</font>
MP1 checks, MP3 checks, <font color="#CC3333">Hero bets</font>, MP1 calls, MP3 calls.

River: (10.50 BB) 7[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(3 players)</font>
MP1 checks, <font color="#CC3333">MP3 bets</font>, Hero folds, MP1 folds.

Final Pot: 11.50 BB

I figured the river had to have hit the opponent to the right of me. He had been calling the whole way, seemingly on a draw and then to suddenly bet on the river when the flush/straight card came kind of signaled that he had something better than TPTK.

I'm pretty sure I was beat at this point. However, what if a blank hit the river, such as the 2 [img]/images/graemlins/club.gif[/img] or something like that? In this case, should I still bet after the two checks, or should I check behind and see a showdown?