FTP 2.5/5 Thin river value bet with KK

[AAismyfriend]

I am new to the table, and villian is unknown, so no stats. However, from what I have seen he appears to be a donk. He limps around a lot and seems passive. I was kind of lost on this river. I decided to bet because I think his most likely holding is just a J, and he could possibly talk himself into calling. Either way, this seemed kinda thin and I'm not sure how good/standard it is. Thoughts?

Full Tilt Poker No-Limit Hold'em, $5 BB (4 handed) Hand History converter Courtesy of PokerZion.com

UTG ($1109.20)
Button ($990)
SB ($1508.30)
Hero ($1000)

Preflop: Hero is BB with K[img]/images/graemlins/heart.gif[/img], K[img]/images/graemlins/spade.gif[/img].
<font color="#CC3333">UTG raises to $10</font>, <font color="#666666">2 folds</font>, <font color="#CC3333">Hero raises to $45</font>, UTG calls $35.

Flop: ($92.50) J[img]/images/graemlins/diamond.gif[/img], 4[img]/images/graemlins/spade.gif[/img], 3[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">Hero bets $75</font>, UTG calls $75.

Turn: ($242.50) 8[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">Hero bets $170</font>, UTG calls $170.

River: ($582.50) A[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">Hero bets $370</font>,