Two friends, Alice and Bob, are participating in a unique race. The race is divided into three segments. In the first segment, Alice runs at a speed of 6 miles per hour for 2 hours, while Bob runs at a speed of 8 miles per hour for 1.5 hours. In the second segment, Alice bikes at a speed of 12 miles per hour for 1 hour, and Bob bikes at a speed of 15 miles per hour for 0.75 hours. In the final segment, both Alice and Bob kayak at a speed of 4 miles per hour for 1.5 hours. Who covers more distance overall, and by how many miles do they differ? Let's think step by step and output the final answer within \boxed{}.
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