For the moderately active person planning a trek such as Everest Base camp or the Annapurna Circuit who lives and works in a city and/or is otherwise limited in their ability to hike outdoors on mountainous our even hilly terrain due to weather or locale.
Sticking with this plan will give you the base fitness you need to not only complete the trek but to enjoy the experience. Time requirements begin at just over 4 hours in the first week and peak at over 7 hours in the final week.