That depends on how close you are to the runners. If at the side of the track then the lens you have should be able to do what you want. Make sure you select a shutter speed of 1/500 second or 1/1000 to freeze the action. If you are taking the pictures in low light, and you are close to the track then a Nikon 50mm F1.8 at about £114 will serve you well. Practice panning with the camera, that is focus on one of the runners before they reach your position, and keeping the focus point on the runner as he moves towards you and press the shutter to take the shot, but still keep the focus point on the runner as he moves past you. It takes a bit of practice but you will soon pick it up.
If you are further from the track then consider the Nikon 70-300mm Lens at £424.
