Breaking in a cam is done with the car in park or neutral and not moving. Believe me this gets things quite toasty. Most manuals recomend having a water hose handy and a big fan in front of the radiator. The first time I did this my car had OPEN headers, at 10:00 at night
And I clearly saw some glowing. The second time 10 years later, I had iron manifolds.
Cruising at speed is totally different, all the moving air cools the engine much better, plus running under load makes a difference too,