This is one of the most frustrating missions in the whole game.
The way I passed it was to first get the three couriers closest to me (The van just at the side of where you take off, the guy on the bicycle, and then the van that speeds off the moment you shoot it, and shoots back at you whenever you are close to it).
This is the easiest bit, the hardest is to get to the couriers at the far north of the land, and then come back with enough fuel. I just flew up to a road that I thought the courier would drive onto; look at the map, when the courier is waiting at a crossroad quite far away, and one of the roads on the crossroads leads directly to where you are, just wait and hope that he comes to you.
I think I was lucky as this is a very risky thing to do, you don't know where they will end up. But that's the way I passed it, just by letting the last two couriers come to me.