Posture: Your spine should be stacked, your scapulae sliding down your back, and your chest should be open to allow you to breathe deeply. Should you exaggerate the whole idea of “shoulders back/chest open” you may arch your back, going past the point of nicely stacked over the pelvis and then you’ll end up putting pressure on your lower back muscles to do a lot of work and take the impact of your steps.
Range of motion: Keep your arms swinging easily, your hands, shoulders, and neck relaxed in this swing. The core should be rotating your ribcage side to side. Pick your knees up by engaging the hamstrings (the often forgotten, underused but overly-tight muscles, since the quads like to do so much work), glutes, and core. Be mindful on not relying too much on your calves and tightening in your hip flexors (the muscles that run across where the femur connects to the pelvis—you can find them by pushing your fingers around the front of your pelvis, you’ll notice them because they will probably feel tight and kind of wonky…aka hurt like they need to be stretched). The hip flexors essentially enable the movement you make while you run, lifting the knee, (they also enable sitting, fyi, which is why those who spend most of their days sitting can have very tight hip flexors), but you have to direct your energy to the larger muscles at work here, balance the work, so that the hip flexors don’t give you trouble. And stretch them like there’s no tomorrow (Crescent Pose in Yoga is a good hip flexor stretch…)
Core: The core is where all your power comes from and it supports your spine. Running is a workout for your abs. Remember this and make sure you stay engaged the whole time (pressing your belly button to your spine with your breath, I’m talking constricting your abs with your exhales, not sucking in your stomach). It takes some practice and diligence to remind yourself of this, but I promise your runs will be so much better, and longer, if you remember to use your core the whole time.