I think we can have a big map and not have the player get lost. Not sure exactly how ArmA works yet (buying it soon), but I sure there is some type of map/GPS system in place. So objectives will be marked on the map, or it could just say head east for 3 clicks, etc.
One thing that would need to be taken into consideration is pacing. So don't make the player walk for 10 minutes to the target without seeing a single enemy patrol or something.
If there is a long way to go then maybe the player could steal a jeep or something to get there quicker. Although, I know you don't want vehicles LS, it does keep the action flowing somewhat, and it could be there as only an option, rather than making players take the vehicle.
On a site note: anyone know where I can get a copy of ArmA in Australia (specifically Ballarat).