Ok i am going to go out on a limb here and say none of you who posted ride dirt bikes competitively. If you are comparing running 26 miles to riding 26 miles down a dirt road or in a field then running is MUCH harder, no contest there. But a motorcycle marathon would be like the baja 1000, where speeds reach 115 mph, much of the terrain is treacherous, you ride at night at speed, as well as during the day, and the race lasts 32 hours. I realize most teams split the riding between 2 riders but this is still quite a feat pedaling or not. I would venture to say there are more people who could run a marathon than there are people who can finish the baja 1000 or even the 500.
True not many motocrossers could ride competitively in the tour de France, but not many tour de France racers could make the main event in a supercross either. The conditioning is much different, but each sport requires equal dedication and training.
I have watched mountain bike races and competed in enduro type races, my sons race motocross and my wife is a mountain biker,
and competitors in all these sports have my respect.
Just my .02