Winning the Middlegame

This game was the 4th I played in the 2010 MN Winter Open.  Day one featured the first three rounds and I scored 2/3.  This game was against an opponent rated quite a bit higher though my goal is to reach 1600.  I misplayed the opening but because my opponent didn't take space and left his king in the center I was able to trade a pawn for a winning position in the endgame.  I felt extremely good having turned the game and a last round game that I would probably lose but that would be very instructional.  However, I just totally blew the endgame!  It's extremely embarrassing to post how I screwed this up.  But maybe it will help someone.  I'm also saving the critical position to replay against the computer a few times until I can win it in my sleep.