hmmm could always change your idea of a difficulty then
how about instead of a set difficulty change it to options
like giving players the choice even mid game to tweak the girls run speeds or if they want the fuel to go down or if they want to increase the number of items on the map [aka fuel cans batterys eta] even item respawning could be in this menu
this way instead of balancing the game yourself you let your players do this for themselfs