Sandbox Gaming Question: How Much Do You Need to Start?

In a chat last week a question came up about how much does a GM have to have pre-planned at the onset of an open, or "sandbox" campaign.

There were several responses, but almost all of them involved using the PC's ability to travel as a factor. Most of the GM's agreed on having the immediate starting area planned out, it was just the mode of travel that was at issue.

Me, I assume the PCs starting out will not have access to expensive horses and will be on foot. I'll plan out the starting town/city and figure that the next villages will be at most a day's walk away. Larger cities will be no less than three day's walk away so the PCs/NPCs could walk there, spend the day conducting business in the city, and walk back in the span of a week.

All I do at that point is flesh out enough of a skeleton to represent those other villages and the city, and have some road encounters, village encounters, and a few dungeons that I can place wherever the PC's decide to go.

Of course, knowing what your players want and how they are most likely to take your adventure hooks helps a lot. I also recommend the d30 Sandbox Companion as a help.

My open question about this is: "How much pre-planning do you think a GM needs when starting a sandbox campaign?"