Nowhere does it say you should keep the car under 60mph. Just 4,000 rpm.. which allows for speeds over 80mph easy.

2003.5 G35 Sedan Desert Platinum/Graphite Premium/Sport/Aero/Nav/Winter

 

I've also heard that periodic engine-braking is a good idea during the break-in period so that you're not only changing the RPM's but you're changing the stresses on the engine too.

<font color=blue>Twilight Blue/Willow, Nav, Aero, 6MT, Splashguards, Clearbra, Tint, Sirius...</font color=blue>

 

That drive should be no problem at all. I went from the Bay area to LA two days after I bought my coupe. Just follow the advice above and you will have no problems in my opinion.
BTW: Just got back from SEMA last week and the drive to Vegas was a blast...I did manage to keep it under 120MPH

My car