There is a skeletal bus service between An Thoi and Duong Dong. Buses run perhaps once every hour or two. A bus (tickets 10, 000d) waits for the ferry at An Thoi to take passengers to Duong Dong.
If you can ride a bicycle in the tropical heat over these dusty, bumpy roads, more power to you. Bicycle rentals are available through most hotels for about US$1 per day.