If that is the case, I am here to tell you that driving from Seattle to San Francisco in a single run is neither impossible nor even particularly remarkable. I make this trip two or three times a year, visiting my California-based family. I-5 is a smooth, easy, comfortable road, well engineered and well maintained. The challenge is not fatigue but boredom, especially through the long, straight sections in the wide-open countryside of Nothern California, where average traffic speed approaches 80 mph. Bring an interesting companion or some good energetic music and you're set.

"The 12th round-Australia motoring event run since the first (1953) Redex, the [largely off-road 18,000km, 14 day] 1979 Repco is now acknowledged as the toughest long-distance rally since the continent-to-continent epics of the first decade of the 20th century. Like earlier round-Australia trials and the London-to-Sydney epics of 1968 and 1977, crews were expected to drive virtually non stop for days at a time with limited rest breaks. Nowadays this would be considered irresponsibly dangerous."

http://repcoroundaustralia.blogspot.ca/

San Jose to Vegas is 523 miles, and doing that in 6-7 hours is quite feasible (although it's entirely possible for it to take 12-14 hours with traffic on I-5 and I-15 if things go wrong)