Hi, sorry for the late reply.
Actually not that far. There are two sand dune areas, Erg Chigaga and Erg Chebbi. The latter one is next to the main road. The other one (this photo) can easily be visited on a 2-day trip from Hmaid or something, in the southeast of the country. Great trip!
Beautiful! How deep into the desert did you have to venture to see dunes this big?