Anecdote time: I currently live in Texas, the closest grocery store is a hour walk. While living in Colorado it was a hour and a half, and in Florida it was 2 hours, all of these where easy 5 minute trips by a car and an excursion by foot.
Reasonable on the interstate but as your driveway (nor the store entrance) are directly on an interstate, there has to be at least some stretch of slower driving bringing the average down, so the speed on the interstate section would have to be much higher to bring the overall average speed to 72 mph.
Let me give a direct example. I pulled up google maps and what is a 10 minute car prediction (2.8 mi) is also a 54 minute walking estimation (2.7 mi) and a 17 minute bike estimation (2.8 mi). Also, 33-43 minutes by bus fwiw
That does change the numbers; 10 minutes by car being 54 minutes on foot sounds a lot more realistic than 5 minutes by car being 2 hours on foot.