texashchman,
I'm in the same favorable conditions as you.
I drive 45 miles into the city of Atlanta, my shift starts at 6PM so I leave the house at about 4PM. I could just hop right onto the freeway but instead like you, take a parallel highway for 20 miles, then the rest is freeway.
The main traffic flow at 6PM is outbound, but I'm going inbound.

I've found that a head/cross wind is worst for MPG.

I get off work at 2AM and the roads are almost deserted.
I take the same last 20 miles of side roads and get zero traffic and take good advatage of this. My commute is about 1hr 15 min.
If I get a particularly good run sometimes I wish I could drive longer.
It's really a game I play with the FCD and have alot of fun at it.

As far as A/C goes:
Some days here in Georgia have already been mid 80's.
Still pretty mild but my car is parked in the driveway sun and gets pretty warm before I leave.
(Will have to change that when it REALLY gets hot!)
So far when I require A/C I'm able to turn it on while going downhill as needed.
That way it doesn't fight the A/C and a hill at the same time.
It keeps blowing cold for quite some time.

Well, I didn't mean to write a novel or anything so take care

Efficient drivers do it better.
1003 miles a tank personal record. 74MPG calculated. HCH1 CVT