Complex sentence: Although the Hills had noted that they had arrived home later than anticipated, the drive should have taken about four hours (178 miles), they did not realize that they arrived home seven hours after their departure from Colebrook.
Split into simpler sentences: Although the Hills had noted that they had arrived home later than anticipated, the drive should have taken about four hours (178 miles). They claimed not to have realized that they arrived home seven hours after their departure from Colebrook.