I think there is another effect at play during a return trip, at least for me. I used to regularly tackle a 900 mile (1,800 mile roundtrip) haul between home and college and it felt longer with each trip.

The trip felt longer as my familiarity with the route increased. I reached a point where I knew exactly how far I still had to travel at any point and the close I got to my destination, the more frustrating it became. I'd feel like I was on the home stretch as I knew my surroundings intimately but I'd still have 1-2 hours left to go.