Why do endurance athletes, such as marathon runners, have a higher proportion of slow fibres in their muscles, compared to fast fibres?

It is more beneficial to endurance athletes to have slow-twitch fibres because they have a greater resistance to fatigue. This means that their muscles can work for longer periods without tire. A greater number of slow-twitch fibres also mean that they have a greater number of mitochondria. A greater number of mitochondria means more ATP is produce, which allows aerobic respiration to, not only occur, but, occur at a faster rate, too. This is beneficial to endurance athletes because respiring aerobically allows them to avoid a build-up of lactate.