Good posture will give you a better running style. Don't slough to slouch, but push your hips forward slightly. Try to keep your body movements in the direction you are running. Your shoulders should stay relaxed, with thumbs resting lightly on partly clenched fingers. Do not worry about your stride length, especially as over-striding (taking very long steps) can be a considerable waste of energy. Relax when you run . you're not trying to fight your way out of a paper bag.
Firstly remember to do it! Always!!
Being serious your breathing rate will increase when you exercise. Try not to think too much about this and do not try to control it too much as your body is very good at selecting a sensible breathing pattern for your intensity of work. As you exercise, your body requires more air as it needs to get more oxygen to the working muscles. You should maintain a pace where your breathing is comfortable. If your breathing becomes very laboured it probably means you're working too hard. Back off and relax a bit more.
Stitches are common if you've eaten too close to your run and also in the untrained. To ease the discomfort, try to breath using your stomach muscles instead of your lung muscles. Don't be afraid to stop to ease the pain. Bending forward will help relax the muscles.
You may find that you get the odd blister. This might well be due to ill fitting footwear. If your shoes are too tight, they will constrict your feet, while too loose could increase the likelihood of friction.
You can decrease the chances of getting blisters by applying Vaseline to likely blister areas (heel and toes) and by wearing good quality running socks.
You might well experience soreness in your muscles especially if you are starting some activity or trying a new activity for the first time. This is the norm and nothing to worry too much about. Some simple stretching will help (see below). However, if the muscles are very sore, this is often a sign of overuse which can easily lead to injury. Take a day or two off and keep up the stretching exercises.
From your early teens your body starts to lose flexibility. By stretching you reduce the risk of injury and increase your range of motion . this means you can run more efficiently.
Ideally you should stretch lightly for 5-10 minutes before running and then stretch some more after running. When you are still warm from training you have a greater range of movement and this is where you can increase flexibility. Before running stretch for 10-15 seconds twice in each position. Afterwards, hold stretches for 20-30 seconds.
These are the muscles at the back of your thigh. Place your left heel on a bench or something of similar height. Lean your body towards your raised foot. Keep your chin stays up and your back straight. You will feel the stretch in the back of your left leg. Hold this position for 20 seconds, then change legs. Repeat three times on each leg.
Put your hands on a wall. Keeping your heels on the ground, lean forward from the hips until you can feel your calf muscles stretching. Hold the position for 20 seconds, release then repeat three times.
These are the muscles at the front of your thigh. Using something for support if you need to, take hold of your ankle and pull the heel of that leg up against your backside. Hold for 20 seconds then change legs repeating three times.
Sit on the floor with the soles of your feet touching. Holding on to your feet, press your arms out against the inside of your thighs. Hold the stretch for 20 seconds, relax then repeat three times.