In the sports circle, we call this the "Sport specific training" phase. It's when you develop the necessary strength base, from general compound activities and then taper slowly into your sport requirements. This way, you won't have trouble with one particular area being a bottleneck, makes you a more complete athlete.

But marathon runners, the only thing I've heard about how they train is, they do progressive overloading as well. Today 5KM, and everyday increase 1KM, until to a point where they can't increase by 1KM anymore. Then they drop by 2KM and then repeat again. They also do high repetition leg workouts like full squats, RDL, leg curls and leg extensions. Oh and lunges.

I've never trained marathon runners, but I'd figure that they could do with a little help on the resistance training side. However, Ethiopian and Kenyan runners are amongst the best but they have barely an ounce of muscle in their bodies. I'm not sure if this is purely genetical though.