The general question about what constitutes a 'hard' race is interesting.

Part of me definitely concurs with Superflyguy - the short races are the ones where you really, really suffer intensely. For ten to twenty minutes, death appears merciful. However, when those minutes are over, resurrection is rapid. Similarly, when I used to run on the roads I always used to say that a 5k was much tougher than a half marathon because you were moving faster than your comfort speed; but 5 minutes after you've finished you find yourself chatting normally and could probably go off for another run.

The long races force you to grit your teeth and stay at it through pain which is at a relatively lower level but for a prolonged period, and recovery times are correspondingly extended.

After 30+ years of running, my conclusion is - notwithstanding my comment on the Grisedale Horseshoe, above - that there is an argument that in one sense all races are equally hard. If you try your best, and adjust your own effort according to distance and terrain, you're operating at your body's limit whether it's a mile along a flat canal bank or 20 miles with 10,000ft of climb. If one event feels easier than another, you're simply not taking it seriously enough.