Marathons are measuring the speed at which you can run a race. If they were a) a measure of your running technique over the 26 miles, and 26 miles was the average distance needed to prove your technique, b) compulsory and c) being non-black meant that no matter how hard you trained, you wouldn't be able to run the 26 miles with good technique, I think it'd be reasonable and fair to let them run a shorter distance.