Why will the Jews at that future time be especially aware of the Sabbath walking limit?
Because the Temple will have been operating for the first three and a half years, the Jews will be consciously observing the Sabbath law and know how far they may walk without breaking it. They will be so aware that most will not dare walk more than about 4,000 feet on the Sabbath, less than a mile.
Les teaches that verse 20 tells the escaping remnant to pray their flight will not be in winter or on the Sabbath. Since the Temple has been operating for the first three and a half years, the Jews will be aware of the Sabbath law and how far they can walk without breaking it. They will be so aware of the Law that Jesus said most of them will not dare walk more than 4,000 feet or something like that on a Sabbath day; it would not even be a mile, so they would not get out of town. He adds that a winter flight would be miserable because they would be walking and carrying their goods.