I feel fortunate to live in Southeastern Pennsylvania. Most of the major parks are grouped in eastern Midwest (Ohio and East) and the Northeast.
In a 3 hour radius I have:
Dorney Park
Six Flags Great Adventure
All coasters at the Jersey Shore
Hersheypark
Six Flags America
Coney Island and others in the Connecticut area
Knoebels
In a 5-6 hour radius (in addition to the above):
Busch Gardens Williamsburg
Kings Dominion
Six Flags New England
Kennywood
Darien Lake
8-9 hour radius:
Cedar Point
Kings Island
Waldameer
Carowinds
12-13 hours
Holiday World
Six Flags Over Georgia
Dollywood.
If given 12 hours, a lot of people could get to a lot of parks from many places on the east coast, but I've got over a dozen parks within 6 hours. And they aren't just run of the mill places. We're talking many of the best and the oldest coasters out there. If you added up sheer numbers of roller coasters, I have nearly 50 in a three hour radius.