My car, when new, could hit 140 mph. Rolling resistance is low and parasite drag is proportional to the square of speed, so the drag on my car at 70 mph is one quarter that at 140 mph, so around a quarter the force at the wheel it is capable of producing. Power = force x speed so with the speed halved the power is reduced by another factor of two, to around 12.5%.
OK so there are fudge factors in there, but it is a ball park figure. Let's add a margin and say that I am using less than 20% of the power to run at 70 mph. OK it was built in Germany, so assume it was made to run well at around 100 mph or about 40% power. Aero engines typically run at around 70% power in the cruise for a sensible balance between speed and fuel use.
So in order to be reliable as an aero engine, my 170 bhp car engine should probably be derated to around 100 bhp. That is starting to look a lot less useful.
I did some work for a motor racing team at one time. They used a very reliable car engine, and of course rebuilt it every race. Working at high power outputs it rarely lasted 6 hours if the race was that long. OK, that is extreme: racing is a tough environment for a car. However it is the end of a continuum of power vs. endurance.