Not a big deal. The GM I used for towing had the same instruction. I followed it, just took the tow vehicle and trailer on a low traffic secondary highway for the 500 miles. We were going camping at the spot which was a bit more than 500 miles away. By the first day it was broken in for towing.
My view is follow the manufacturer's instructions. I always break in my vehicles according to what the maker says and then some. So did my dad.
I also don't run the vehicle at a constant speed while breaking it in, and back off the throttle every so often to change rpm.
It pays off in the long run.