redman:
Sorry, mate, but I have to disagree with your claim that a horse has to have competed in a race the length of the National.
For example, I'm fairly sure that there isn't another race of that length in the racing calendar, not in the UK anyway, and the nearest you'd get is either the Eider Chase at Newcastle, the Scottish National at Ayr, or the Midlands National at Uttoxeter, all at 4 miles.
There have been many National winners over the years which had hardly run in excess of 3 miles, let alone an extra mile and a half. It has been held for many years that a class two and a half mile horse, for some reason, is an ideal type for the National.
Nowadays, there are more and more class animals contesting the race since the fences were modified some years ago, and the race is increasingly won by horses who have contested Pattern races etc as opposed to your donkeys which used to win yesteryear when the race was nothing more than a lottery - as in the first ever winner of it.