[Tiva]

The population density and need for food makes the migration faster. As long as only humans carry the virus and it does not mutate to animals that is a pretty good rate of migration because they need to feed (assuming 2 brains a day). Since they don't sleep or need to rest in any way they could migrate faster than even birds could if they crossed water.
And I did the typical rate of slow decomp for the US, 2 to 5 years for a full skeleton, unburied and left exposed to the elements. So for them to be able to walk they would have to feed less than the needed amount for about a year before their leg muscles disconnected and they couldn't walk anymore. Also figure in that if they started attacking each other that the rate would go up since they are injuring themselves and exposing more open flesh for bacteria to feed on.
I want to live in a geo dome up in the polar areas. Green house with sun 6 months of the year to grow fresh food and the other six to work on a cure or play shoot the zombie.