Qual é o segredo das estradas valirianas?

Acho que encontrei o que você está procurando:

De acordo com o wiki:

"At the start of his journey to Volantis Tyrion Lannister takes the opportunity to have a closer look at the Valyrian road that he and Illyrio are traveling on. It runs as straight as a spear to the horizon. It is a ribbon of fused stone raised half a foot above the ground to allow rainfall and snowmelt to run off its shoulders. Tyrion muses that unlike the muddy tracks that pass for roads in the Seven Kingdoms, the Valyrian roads are wide enough for three wagons to pass abreast, and neither time nor traffic mar them. They still endure, unchanging, four centuries after Valyria itself had met its Doom."

As estradas são feitas de pedra fundida que é construída de forma a permitir que a chuva e a neve escorram rapidamente pelos lados.

Podemos argumentar que isso é um fator enorme em sua preservação e sua condição.

Quanto a mágica, o wiki não está claro, mas de acordo com outra página do wiki O aço valiriano é mágico. Portanto, existe a possibilidade de alguma magia ter sido usada para criar as estradas.

As estradas valirianas são feitas de pedra fundida e levantadas do chão para permitir a drenagem. Eles geralmente são largos o suficiente para que 3 vagões passem lado a lado e sejam extremamente retos.

During one stop, he used the time to have a closer look at the road. Tyrion knew what he would find: not packed earth, nor bricks, nor cobbles, but a ribbon of fused stone raised a half foot above the ground to allow rainfall and snowmelt to run off its shoulders. Unlike the muddy tracks that passed for roads in the Seven Kingdoms, the Valyrian roads were wide enough for three wagons to pass abreast, and neither time nor traffic marred them. They still endured, unchanging, four centuries after Valyria itself had met its Doom. He looked for ruts and cracks but found only a pile of warm dung deposited by one of the horses.
_{A Dance with Dragons, Tyrion II}

Não há informações sobre como as estradas são feitas, mas como não há rachaduras nelas depois de quatro séculos e elas são fundidas em pedra, não é improvável que sejam feitas de pedra derretida. Especialmente considerando que o fogo do dragão é quente o suficiente para derreter a pedra e que os valirianos tinham dragões, é claro.

Stone does not burn, Harren had boasted, but his castle was not made of stone alone. Wood and wool, hemp and straw, bread and salted beef and grain, all took fire. Nor were Harren's ironmen made of stone. Smoking, screaming, shrouded in flames, they ran across the yards and tumbled from the wallwalks to die upon the ground below. And even stone will crack and melt if a fire is hot enough. The riverlords outside the castle walls said later that the towers of Harrenhal glowed red against the night, like five great candles...and like candles, they began to twist and melt, as runnels of molten stone ran down their sides.
_{The World of Ice and Fire, The Reign of the Dragons: The Conquest}