Complex sentence: They then march cross country against the clock, increasing the distances covered each day, culminating in what is known as Endurance: a march with full equipment scaling and descending Pen y Fan in 20 hours.
Split into simpler sentences: They then march cross country against the clock, increasing the distances covered each day, culminating in what is known as Endurance. This is a march with full equipment scaling and descending Pen y Fan in 20 hours.