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.
One of the classic hunting methods that humans evolved to handle is to just chase after an animal with less endurance until it collapses of exhaustion. Humans are designed to run farther than other animals, and deal with the heat better.