Is it true the way you break an engine in is the way it will perform?

I just bought a used Durango with a 5.9 in it. I would think it should have alotta power, but I'd swear it was a V-6. Works good, just not overloaded for power. Also how durable are the 5.9 engines...I've never had a big V-8, and it's got 300 000km on it now. Just gonna be a plow truck/second vehicle, but how long will that old motor last? Thanks