[Gurthang]

I think Aragorn had a pretty good strategy. They stayed off the main roads, and went through the marshes. This did slow them down, but the Nazgul didn't like water, or was that deep water... anyways, they weren't caught until they reached Amon Sul.

Truthfully, after that point in the journey, going anywhere but the main road would have taken way too long to be a plausible choice, and that's why the Nazgul caught them. At that time they were on the path the Ringwraiths would look on, and so were caught.