Approximately how long was the entire National Road?
The National Road was approximately 620 miles long.
The National Road was the first major road project of the federal government intended to connect the eastern part of the United States and the (then) western frontier.
The road was built between 1811 and 1837, and stretched from the Potomac River at Cumberland, Maryland to the Ohio River at Vandalia, Illinois.
It was rebuilt and improved in the 1830's and was the second road in the US paved with the new "macadam" process, developed by John McAdam of Scotland. We now call that process "blacktop".