While the site indicates that several million Americans will be able to reach the path by driving four hours or less, it looks like millions of others will have to drive much longer. If you live in NYC, for example, you'll likely have to drive for about 10 to 12 hours, while folks in southern parts of California and Texas will have to drive for even longer than that. The driving time estimates are based on average highway speeds, although traffic on the day of the eclipse could end up slowing things down a bit. The point, though, is to give you a good idea of how long it'll take you to get there so you can plan accordingly.
Speaking of planning accordingly, GreatAmericanEclipse.com also has several additional maps highlighting the best places to view the eclipse (you're in luck, Carbondale, Illinois!), breakdowns for each state along the path, and even a map showing the local time when the eclipse will take place in the sky in several spots across the country (shown below).