To break in an engine properly, you must "heat cycle" the internals. Basically, it means drive around normally if you are in stop and go traffic. Do not be afraid of high RPM, but don't spend all your time there. As you run high engine speeds and then low speeds, you will heat and cool the rings, etc. This is important for proper break in.
On the highway it is a different story. Spending all your time at the same engine speed is BAD BAD BAD. If you have to do an extended highway drive in the first few thousand miles or so, you should run the engine at elevated speed occasionally. What I did was every 10 - 15 minutes, I would drop to 4th gear for 30 seconds or so, then back into 5th. This would run the engine faster (and hotter) for a short amount of time, then run slower (and cooler) for a little while. Heat cycle, in other words.
If you read your owners manual, it basically says to avoid highway driving in the first xxx miles, and otherwise drive around normally. I have basically given you a way to get some highway miles in during break in by explaining why that advice is given by the manufacturer.
