Heat is it for me, I leave for work and it is 70° breathing is easy. On the way home it is 90°+ and the wind feels stronger, air feels heavier to cut through. I have about the same commute 20 miles each way. I can't do the 40 miles a day all week, only for maybe 3 days a week. The other days I take a train and do about 28 miles round trip. I dont think there is enough hours a day for me to do 40 miles a day all week. My body just cant handle it yet.
My body makes me eat before my ride home, round 4pm and I leave the office at 5. If I don't I usually have to stop half way and get a second drink.