5/10 River OOP - Blocking help

[AEKDBet]

Party Poker No-Limit Hold'em, $ BB (6 max, 5 handed) FTR converter on zerodivide.cx

Button ($1002.35)
SB ($421.50)
BB ($1199)
Hero ($1104)
MP ($1306)

Preflop: Hero is UTG with A[img]/images/graemlins/spade.gif[/img], A[img]/images/graemlins/heart.gif[/img]. SB posts a blind of $5.
<font color="#CC3333">Hero raises to $30</font>, MP calls $30, Button calls $30, <font color="#666666">2 folds</font>.

Flop: ($105) 2[img]/images/graemlins/diamond.gif[/img], 7[img]/images/graemlins/diamond.gif[/img], K[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(3 players)</font>
<font color="#CC3333">Hero bets $80</font>, MP folds, Button calls $80.

Turn: ($265) A[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">Hero bets $250</font>, Button calls $250.

River: ($765) 8[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(2 players)</font>
Hero checks, Button calls $642.35 (All-In)...

Now I hate myself for checking. WTF am I supposed to do, block for 275 and fold to an all in.

(New villain, never raises preflop.) I swear I seriously thought about moving in on that turn, but I told myself that's just ridiculous.