Strong, healthy legs are essential for running, and leg training for marathon runners should include not only strengthening, but stretching exercises. Running has many benefits, but increasing your flexibility is not one of them, and many running injuries stem from tight legs.
Even if stretching isn't your thing, it's worth making time for these "big three":
Calf stretch: Place your forearms or hands against a wall. Bend one leg and place that foot on the floor and step the other leg straight back, with your back foot pointing straight ahead. Bring your hips towards the wall, while keeping your back foot flat on the floor. You should feel a stretch in your hamstrings. Hold an easy stretch for a count of 30.
Hip stretch: Sit on the floor with the left knee bent and left foot flat on the floor. Bend the right leg and place the right foot on top of the left knee; your right knee should point out to the right side. Support yourself with your hands so you don't fall backwards. You should feel a stretch in the outer part of your hip. Hold for a count of 10, then switch legs and repeat on the other side.
IT (iliotibial band) Band stretch: Stand up straight and cross one foot over the other. Bend forward from the hips and reach your hands towards the floor. Keep your feet flat on the floor and don't move them, but engage your leg muscles as if you were trying to move your feet closer to each other. This action should engage the IT band along the outside of the back leg. Hold for a count of 10, then relax, and engage the muscles again. Stand up, cross the opposite leg over, and repeat on the other side.