I'd pack snacks and download an audio book and let him drive while I zone out with headphones on. 1pm on a normal Friday would be slow, but on a holiday weekend I'd expect a few stretches of only inching along for a few miles along the Beltway (possibly most of it, depending on accidents) and then crawling on various portions of 66. Meaning 3-4 miles might take you an hour.
It all depends on if it's raining (double your travel time) or if there are accidents (at least 50% longer than normal traffic).