The Ugly Win

I am playing against a 1708 in a USCF online tournament and have the black pieces. Go to move 40. I would ultimately go on to win by milking the clock. Kenzie later pointed out to me I could guarantee victory with a better understanding of rook pawn endgames and a lot of patience (his game ended before mine) however, I had a much clearer chance to win earlier in the game. What mistake do you think I made and what move should I have made and why?