I know a couple of truckers (OTR) who had their motors not replaced
but over-hauled and each of them had over 1 mill. miles within 7-11 years driving.
They told me. You maintain your equipment, your equipment will maintain you (will keep you OTR).
600, sounds about right. M / L
Most semis spend several million miles on the road before their day is done. They have parts that are meant to be replaced and are readily available. A truck with 600k mi wouldn't scare me, but be aware at that mileage it might be time for an engine rebuild.