Actually, most modern airfields have runways that have a length of 3,360 meter (3,4 kilometer)
A big airplane like a 757 needs a landing strip of 1,8 kilometer to land safely. (minimum requirement under normal weather conditions by an experienced pilot)
An A380 is said to need about 2,4 kilometers to land safely.
note that these figures are for NORMAL conditions and for safety reasons they are usually much longer. As a rule of thumb a take-off and landing strip needs to be long enough so that an airplane would theoretically be able to lose all engines except one and still be able to take off or abort the flight. So usally they're more than 3 kilometers even if under normal circumstances no plane actually needs all that space, it's for the abnormal situations to make them safer.
Point is, even if you'd make a runway all across the whole region it would still not be long enough for a big plane to land, and than you don't even have things like passengers terminals and taxi-lanes.