Welcome to the club. I've got an 06 Route 66 Edition and the electronic seats are terrible, and I even have the built in lumbar support. My hips and siatic nerve can only make it around the 50 mile mark like you. I've tried different types of back supports with little comfort. From what I gather from PT car shows is that when Chrysler redesigned the Cruiser in 06 they put in really poor seats with no support. These seats were a huge step down from earlier models. Apparently they put better seats in the newer models. Right now I'm searching for wrecks ( Non 2006) that I can pull the seats out of. That might be a solution for you too. Good luck.
I have a 2007 Limited Turbo with the leather/suade seats and I found them to be great for the 100 mile trip we took in December. I have a blown disc in my back but the adjustable lumber support and power seat and tilt wheel means I was really comfortable. I test drove 8 PTs from 2003-8 before I found mine with 51K miles (Tangering Pearl with sunroof) and I thought the 06+ had more comfortable seats because of the full power adjust, not just the up and down sillyness of the older ones. I like the older arm rests better however, but liked U-Connect and the Boston stereo and the Gauges of the newer model. Just my .02