وَجَعَلْنَا بَيْنَهُمْ وَبَيْنَ الْقُرَى الَّتِي بَارَكْنَا فِيهَا قُرًى ظَاهِرَةً وَقَدَّرْنَا فِيهَا السَّيْرَ ۖ سِيرُوا فِيهَا لَيَالِيَ وَأَيَّامًا آمِنِينَ (18)
(34:18) We placed other prominent towns between them, the towns that We had blessed and had set well-measured stages between them. *31 Move back and forth between them, night and day, in perfect security.
*31) "Blessed habitations": the lands of Syria and Palestine, which have been generally mentioned in the Qur'an by this title, as for instance, in AI-A'raf: 137, Bani Isra`il: 1, Al-Anbiya`: 71, 81. "Conspicuous habitations": habitations situated on the highway and not inside the country. It may also mean that the habitations were not very far apart but contiguous so that as the outlying areas of one habitation came to an end those of the other started coming into view.
"Set...distances" implies that from the Yaman to the borders of Syria the whole journey passed through inhabited lands, and the distances between one station and another were known and determined. That makes the distinction between the journey through inhabited land and the journey through uninhabited desert area. In the desert the traveller continues to travel as long as he wills and halts when tired. Contrary to this, in settled areas as the distance between one habitation and the other is well known, the traveller can plan beforehand when he would break his journey, where he would have his midday rest and when he would stay for the night.