The value of a pawn

I just finished my first correspondence chess game - with a loss. While studying annotated games of grandmaster-level players, I realized that every pawn is important to them. But so far, I was not too concerned about the safety of my pawns and would sacrify them unnecessarily to "improve" my position or simply by careless moves. In fact, I am usually happy not to loose my pieces. My new (and limited) experience with very slow games, however, taught me, that every pawn is important, particularly when games reach the endgame with only few pieces around. In the game below I play black against mike1970 at ChessWorld and resign after giving away my rook, although I've been down a few pawns before that, anyway. In the future, I will try to care for all members of my "army"!

1.e4 e5 2.Bc4 Nf6 3.d3 c6 4.Nf3 d5 5.exd5 cxd5 6.Bb5+ Bd7 7.Bxd7+ Nbxd7 8.Bg5 Bc5 9.O-O O-O 10.a3 Qb6 11.b4 Bd6 12.c4 e4 13.dxe4 dxc4 14.Nc3 Ne5 15.Bxf6 gxf6 16.Nxe5 Bxe5 17.Rc1 Rfd8 18.Qg4+ Kf8 19.h3 Qd6 20.Nd5 Qe6 21.Qxe6 fxe6 22.Ne3 Bb2 23.Rxc4 Bxa3 24.Rc7 b6 25.b5 h5 26.Rh7 Rac8 27.Rxh5 Ke7 28.e5 Rc1 29.Rh7+ Ke8 30.exf6 Rxf1+ 31.Kxf1 Rd7 32.Rh8+ Kf7 33.Rh7+ Kxf6 34.Rxd7 a5 35.bxa6 1-0