If you head out now to take some air, whether walking or jogging, you could settle easily into a fairly effortless rhythm. However, to pick up the pace for a brisk walk or a more energetic run, you'd make a conscientious effort - one that may initially feel a bit uncomfortable, or at the very least feel...well like an effort. The nerve impulses travelling from brain to muscles in the legs, butt, arms and shoulders would be fired more rapidly. The world around you would be passing by more rapidly and providing your brain with a bit more to process. There would be awareness of changing states within your body e.g. temperature increase, increased breathing rate & depth, twinges around a joint. All these mean your brain is coping with more and it needs to learn to juggle these activities whilst running. An increasing body of research suggests that runner performance is limited more by mind/brain/neural function than by muscle physiology. Running is as much a learned and developable skill as playing a musical instrument. Practice is important.