exercise increases the density of capillaries through which the blood flows to and from the muscles.
exercise increases the production of haemoglobin, which carries oxygen in the blood.
put both of these together and you get more oxygen, a lot more oxygen, being delivered to the muscles.
exercise also increases the size of the heart chambers, increasing the volume of blood that can be pumped at each stroke.
exercise also improves the efficiency of the heart muscle so that each stroke is stronger than before, improving the pumping action which also increases the blood volume pumped at each stroke.
exercise also improves the neuro-muscular pathways that send nerve signals from the brain to the muscles telling them to contract. this makes the signals route more efficiently so the signals get more accurately and more quickly to the correct groups of muscle fibers for the task in hand.
exercise also increases the production of certain enzymes which you can think of as chemicals that enable specific chemical reactions to take place. this improves the number of and efficiency of those reactions so that, at a very basic cellular level, things like converting glucose into ATP for use as muscle fuel, and converting lactate back into glucose again, happen more frequently and more efficiently.
most of these enhancements happen most beneficially when you exercise at an easy, aerobic pace. lots of slow, easy miles, that doesn't place too much stress on the system, is the key to maximising these benefits.
therefore, you get them by running slowly. and when you get them they increase your potential to do other training which in turn improves your race times.
there is no short cut to success.
cheers.