Complex sentence: Many people use Lake Placid as a base from which to climb the 46 High Peaks in the Adirondack Mountains; those who complete these climbs may join the Adirondack 46ers.
Split into simpler sentences: Many people use Lake Placid as a base from which to climb the 46 High Peaks in the Adirondack Mountains. Those who complete these climbs may join the Adirondack 46ers.