As a 9-time marathon runner who also does half marathons, 10Ks, and 5Ks, and help you improve your running performance through speed interval training, running drills, and even resistance-training tailored to running.

Weightlifting is important for building muscle.  Stretching is important for keeping your muscles from being too tight and improving range of motion.  Fitting both in our schedule is tough, so power stretching is combing stretch with resistant training.  

Resistance-training is important for building muscle and bones.  As we age, we lose muscle mass and bone mass, which makes us weaker.  While using dumbbells and barbells are the main types of resistance-training that people  utilize, medicine balls and kettlebells are other methods of resistance training.  I have even helped elderly clients drastically increase strength.