Brilliant that you think this through! Bravo!
You do have a bit of a concern. Even domestic ducks that don't quite fly, they can (what I call) do wing-assisted jumping. So sometimes they can clear a three foot fence.
What you can have in your favor, though, is training them. If you can get them to associate really good times with staying close to you, it will help keep them with you when you walk around.
I take my ducks out for walks around our 0.8 acre place. The first time we walked across the yard, I was so nervous! But I had spent countless hours socializing them, giving them treats, etc. And they stuck with me.
I do need to pay attention to them. If I get lost in a book, even standing near them, they will begin to wander off. I put myself between them and areas they tend to wander toward. That's enough, for the most part. I have gotten distracted, and ended up having to trot to get behind them and herd them back closer to the house.
You also want to keep an eye to the sky to make sure raptors don't take a bead on them.
But I have found that if I am near the flock, raptors just keep going.