Standing forward bend or Uttanasana is a good stretch for hamstrings and lower back. It is good for runners, specially because they have tight hamstrings. This lets the runners go an extra mile. Also if you have a stationary/sedentary job and you sit in front of your computer for long hours, it is good to get up and stretch your lower back.
To do Uttanasana, stand in a tadasna or the mountain pose and slowly bend forward. While doing this exhale. Bend forward from the hips and not your waist. Depending on the level of your flexibility, you can touch your palms on the floor or fingertips. More options and variations are taking the hands behind your ankles. If you are not very flexible, go to a point you can, fold your hands from the elbows, interlock forearms and just let the body hang forward. While doing this, make sure, the heels are pressed down but your weight is shifted forward on your toes so that the tail bone is pointed towards the ceiling. Try to extend the knees straight but do not lock your knees. Turn your upper thighs slightly inwards. Bend the head down. Do not try to look ahead. If you cannot keep the knees straight, you may bend them. Slowly as you will become more flexible, you will be able to keep the knees straight.
Remain in this posture for sometime. With each exhalation try to bend more forward. Make sure that the legs are active and feel the stretch.