Not good. The cylinder walls and rings do a lot of wearing in during this time and those small particles of metal are floating in the oil. Did you do an oil change after the first hour or so of running? If so, it won't be as bad, because you would have gotten alot of the metal out of the motor. I always change the oil & filter after about the first hours of running, sometimes less. When draining the oil the first time, it will look like metallic silver paint.
Lee
1966 Barracuda, 340 4 speed