Back in The Good Ole Days - back when rings and cylinders (and engines in general) were made out of relatively low-tech materials and tolerances were a lot looser - extended break-in periods were necessary. Not so much nowadays. From what I've seen, breaking in an engine is almost a thing of the past. You still want to baby it for a few hundred miles, but it's not the big deal it once was.
IMNSHO, most modern engines are pretty well "broken in" by the time they have 3-4 hours on them. Your engine is probably ready for whatever you can throw at it.
But I'm just another one of those "opinionated yahoos on the Internet." Definitely follow Nissan's break-in procedure. As IDZRVIT points out, the engineers who designed the engine should know what's best for it.
