Or HaChaim on Exodus 8:24/אור החיים על שמות ח׳:כ״ד

1 לא תרחיקו לכת. "Do not go very far." Pharaoh meant: "not as far as the three days you have requested." His argument was that if Moses was afraid the Egyptians would stone the Israelites if they slaughtered animals the Egyptians regarded as sacred, there was no reason to travel a distance of three days to escape that particular danger. Apparently, Moses did not make an issue of this at this time; he reasoned that the Israelites' major purpose was to get out of the urban areas, no matter how far. If Pharaoh would pursue them, Moses was certain that G'd would deal with him. At that point Moses wanted only to ensure that Pharaoh would not have a reason to renege on his promise; this is why he warned Pharaoh not to deal deceitfully with him and the Jewish people. He emphasized this by use of the word רק. Moses meant that though they would travel for a distance of less than three days the important thing for them was to get out of town for a while.