It's silly to argue over whether humans were "designed" to run that far. There are some things we definitely can't do, like jump fifty feet in the air or outrun a cheetah or stay underwater for two hours. But running a marathon is nowhere near the limits of human capability -- pretty much any able-bodied human being under the age of fifty can get there with sufficient training. I've never run a marathon but I've done a half and I'm planning a full one in the next few years. Of course the mental component is a lot easier for the happy-just-to-finish crowd like me than it is for anyone hoping to actually finish at the front.