Now I’ve plotted this: http://tinyurl.com/3d6uwb

This is a drive from Fairbanks, Alaska to Key West, Florida. Then to San Diego, California, and back over to Labrador City, Quebec. Then back to Fairbanks. Repeat this sequence about 6 times. And check “Avoid Highways”.

The result is over 108,000 miles (~4x circumference of the earth), and would take 110 consecutive days of driving without stopping. If you drove 8 hours a day, it would take you almost a year. The Google Maps directions include 5,022 steps (And you wind up back at Fairbanks city hall).
I’ll be the first to acknowledge I could have made an even longer trip by just going back and forth between Fairbanks and Key West. I could have made a more interesting pattern by making tight zig zags across north america. So this is kinda in between.

I still wonder what the longest possible Google Maps trip is. If they allowed you to plot a drive on the Pan American Highway, THEN we’d be talking some serious mileage.