What are the physiological benefits of long endurance rides?

Long rides that are considered endurance pace focus on building an aerobic energy system. Specifically, you can expect to increase size and strength in your slow-twitch muscle fibers. You will increase capillary density, so you can absorb more oxygen from circulating blood. You will increase mitochondria density, the structures within the muscles cells that produce ATP or energy, so you can produce more energy, quicker. You will also increase stroke volume, or the amount of blood each beat of the heart can pump so you can circulate more oxygen to working muscles per heart beat. Finally, you will improve temperature regulation and increase respiratory endurance. There are clearly a number of benefits at riding at an endurance pace below threshold. So it is an imperative part of any successful training program.