Just roll the window down now and again. Stop when you need to and take a piss break, have a little stretch and get the blood flowing. I can drive for a couple hours at a time but I eventually need to get out and piss and just walk around.
Just got back from a little road trip myself. There was a huge blanket of fog and we couldn't see 10 feet in front of the vehicle. The fucking TomTom GPS was way different than my Magellen GPS so it really fucked me up. Surprisingly got right to my destinations without any difficulty though. Plan ahead, chew some gum, put some good beats on your MP3 or a blank CD, etc. Helps to have someone driving with you to keep you company.