Endurance, the building of durability for the chosen race distance.
In training, a progressive overload or periodised program helps achieve optimal performance improvements. Long Slow Distance (LSD), such as running 90-180 min, biking 3-7 hrs. avg < 75% MHR, increases muscular endurance. Pace should be conversational. You should negative split without pushing pace. Far to often athletes do endurance training at to high an intensity to achieve the desired results.