Woah, this is a big subject! Running success (which to me means completion without injury and enjoyment, to others it's about time) is dependent on posture, alignment and running style.
Be aware of how you run. Tune in and 'body scan' - how does it feel from the toes all the way to the top of your head - perhaps your left and ride side feel different. Make adjustments, this might be how your foot is planting ie heel, mid or forefoot or running on the other side of the road to counter the camber influence!
Every time your foot strikes the ground during running, a vertical force approximately equal to 7-9 times your body-weight is transmitted through your and due to Newtons Third Law states that for every action there is an equal and opposite reaction reflected back up the body (ref run3d.co.uk) It's not just acceleration but countering deceleration. And then there's the weight contribution to force and on your joints and the effect of weight on your run time over the course of a marathon - a whole 'nother subject!
Thinking through ideal running posture and alignment here are a few of my thoughts and observations working from the feet upwards:
feet parallel (2nd toe leads, feet shouldn't flick out to sides), feet level (as opposed to over pronating or supinating)
knees over ankles (knees not dipping in, or out)
pelvis stablised and level (strengthen those hip stabilisers ie glut med), extend the hips properly (use glut max don't allow the hamstrings and calves to overcompensate as they will get overused and susceptile to injury and/or cramp),
engage core (navel to spine at 30%, pelvic floor engaged to 30%) ribs down not flared,
shoulders relaxed and chest open (not hunched into neck) and stablised
neck long, look ahead (not down), and not up which happens when using accessory respiratory muscles to assist in respiration when it's tough to breathe
relaxed arms and hands thumbs forward
Over 26.2 miles you're going to be on your feet for many hours, think how many foot steps that is on each foot. Your foot has 26 bones, 32 joints, hundreds of ligaments and muscles tendons that attach into and around the foot and ankle and from the lower leg. There's a huge amount going on and we often neglect our feet but a seemingly minor problem in either foot can cause major problems as the miles increase especially running on concrete roads. Get all the joints moving and flexible, massage your feet, move them in all directions, splay your toes (tricky for most), create those foot arches, stretch the foot fascia underneath the feet, strengthen the muscles of the feet (use your toes to claw 4 sheets of toilet paper towards you (try it!). Come up onto your toes and down again how does it feel? Can you balance without the feel falling to the inside or outside?
Breathing should not exhaust the accessory respiratory muscles of the neck and shoulders but should be into the low back ribs and if you breathe in through your nose and out to pursed lips you will feel your core start to engage and give you strength and concentrate on exhaling CO2 than