The book says to stay below 40 for the first 500 km (about 300 miles). I was told that the book was written for the older engine, and that the newer Electra X motors could be broken in at 55 mph, which meant less cars passing me in the curves and against the double-yellow lines. I did try to vary the speed and give it chances often to cool off.
After the first 300 miles, I rode between 55 and 60 mph. I have just over 700 miles now (1100 km) and it is officially broken in. I found a long straight stretch of country road, and tried out 75 mph for a bit, but I won't be doing it often.
I can now get on the interstates and not be a hazard.