Why will the Jews at that time be so careful about the Sabbath law?
The Jews will be so careful about the Sabbath law because the Temple has been operating for the first 3½ years, making them acutely aware of the law and how far they may walk without breaking it. They will not dare walk more than about 4,000 feet, not even a mile, so they would not get out of town.
Les explains that since the Temple has been operating for the first 3½ years, the Jews are aware of the Sabbath law and how far they can walk without breaking it. Jesus told them to pray that their flight would not be on the Sabbath, because their law-consciousness would limit them to about 4,000 feet—not even a mile—so they would not get out of town. He also said to pray it would not be in winter, since they would be walking and carrying their goods, making the flight miserable.