I look at it this way: metals have a certain 'endurance limit', where the metal component will theoretically last forever (i.e. "infinite load cycles") if the stress on the component does not exceed the endurance limit. If you exceed the endurance limit, the number of allowable load cycles decreases, exponentially depending on how much you exceed the limit, and of course more immediate failure can occur if you flat out exceed the ultimate yield strength of the material.
In terms of the car: the engine is advertised at ~300HP at some engine speed, but I would venture a guess that at ~300HP output you are above the endurance limit of the engine (I'm oversimplifying because the engine is made of so many parts), and therefore you will not get infinite life out of the engine at 300HP. However theoretically if you ran the engine continuously at ~50HP output, it should live forever (again theoretically assuming proper lubrication and other maintenance to keep it circulating and breathing healthily).
<----- Is a mechanical engineer