Lost bishop of opp colors endgame..

  So I lost a bishop of opposite colors endgame from this position.  I was wondering if Black's passed pawns should have given win a winning edge, or if i simply misplayed this endgame.

To get into the endgame, my opponent, i felt incorrectly, moves to Rf1, letting me trade into this opp-color bishop ending, which "usually" is drawn.  Perhaps he really knew his outside passed pawns were that decisive, or perhaps he thought I would have to play 31.Rxf1?, allowing ...Bxf1, where after 32.c5 Bc4, the d pawn falls also.  But 31.Kh2 avoids that line.  He trades anyway.  So I'm in bad shape, but I've got theory on my side, right?  What is white's plan of defense here?  Maybe if i had played my a pawn up to a5 when i had the chance to, say on 39.a4 then a5 next move?  That might have sealed the deal, since my bishop could concievably bounce between defending b4 and targetting the diagonals in front of black's passed pawns, while my king sat in front of them.

Any thoughts at all on how this endgame was or should have been played?  Thanks!

his pawn advantage on the side queenside was enough to keep your king away from your pawns that needed his help. there were too many pawns for your bishop to worry about.