Quite a few people have told me about getting out to the highway to get a bus.
The most recent one said he walked, but wished he got a taxi, not because of the distance but because it was a very hot day.
The taxi fare for that distance should be BZ$6 (US$3), but airport taxis might charge more. Another person said he paid something like BZ$10 to $15.
Others said they started walking and were picked up by cars heading out to the highway.
As you approach the intersection with the highway, Belize City is on your right. Orange Walk is on your left, so you have to cross to the other side of the highway and wait. When you see a bus coming, wave your arm up and down to get it to stop; Express buses will not stop. The sign in the front window of the bus might say Corozal or Chetumal, but the bus will stop in OW.
This is a common way to get a bus in Belize.