Ascending Denali, especially via the West Rib Route, requires a high level of physical fitness, as well as climbing and mountaineering experience. On the first day on the trail, you’ll have 100+ pounds of kit in your pack and sled. After Camp 1, you will be carrying a 40-60+ lb backpack and pulling a 30-50 lb sled on the lower glacier for up to four or more hours a day. Higher up you will need to negotiate fairly steep terrain with a 50+ lb pack.
A good combination of aerobic and strength training is needed to prepare for an expedition. Summit day can be long and though your pack will be fairly light, people often call it the toughest day of their lives. If at any point you fail to demonstrate sufficient skills or fitness to climb higher, you might create an unacceptable risk to the guides and to the team, leading to the cancellation of the climb.
You should know how to tie knots, know basic ice axe techniques (including self-arrest), have crampon skills (French technique), and know how to handle harnesses and prussiks/ascenders, and how to affix them.