Frankly I don't see any issue with the name and if I ever manage to run that distance I sure won't be bothered by it being called a half-marathon.
If anything, this race has the word "marathon" in the name, which takes it to a whole different league than the 5k, 10k, etc.
As has been pointed out, non-runners rarely know the distances anyway, and runners know what an accomplishment the race is.
Just the same as a 100-mile run can still be a called an ultra-marathon, and ultra runners don't seem to have an issue with that, even though the distance is not comparable to a marathon at all.
A half-marathon is exactly what its called - half the distance of a marathon. Not to mention the fact that it doesn't require extra history/ pronounciation lessons. 