He wasn't trying to redraw the road, he was drawing the chasm line.
All Aons use the base Aon known as "Aon." It used to look like this:
This Aon is based on the geography of Arelon. The top line is the Sea of Fjorden, the right the Atad Mountains, and the dot is Lake Alonoe. See the map of Arelon:
What's important is that during the Reod, a chasm opened up in southern Arelon in the Tii province. By the compass rose on this map, in Sel (the world on which Elantris takes place), south is called "Toa." The chasm resulted in the geography of Arelon changing so drastically that now an additional line needs to be drawn for the chasm in all Aons. Also note that the chasm line isn't straight east/west. It dips a little towards the south.
The city of Elantris plus its four satellite towns are built to resemble the Rao Aon, which looks like this:
Where the sort of square in the center is obviously the base Aon. The actual city of Elantris is shaped the same as Rao's sort of square, and it has four circular satellite towns. We know from the text that one of those satellite towns is Kae, which is where much of the book takes place, and not coincidentally, that is the name for east on the compass rose on the map. In fact, the compass rose also seems to contain the Rao Aon.
Therefore, Raoden was running towards Toa, the southern satellite town, in order to draw the chasm line to the south of the base Aon, running in a northwest to southeast direction.
The epiphany he had was the fact that Elantris resembles the Aon Rao. Once he knew that, he knew the exact placement of the chasm line from common knowledge of the chasm that opened up in the Tii province, and because he'd already discovered that any Aons he attempted to draw required that same line in order to function.