Increasing stamina while in the office
The best way to improve at riding is to ride -- but many people do have limitations on their time that make it difficult to ride as much during the week as they would like. 

At one end of the spectrum, there is some amount of almost all fitness-building activity that is transferable to cycling fitness. Cycling is mostly an aerobic sport, so vigorously climbing stairs or vigorous lunch walks can help. Parking or getting off the bus a few blocks away from work and then walking vigorously the rest of the way can help your overall aerobic fitness. 

At the other end of the spectrum, you may want to investigate purchasing (or borrowing) a magnetic or fluid trainer on which you can place your bike so you can train at home. Many people who live in cold climates who do not wish to ride outdoors or after dark in winter either train at home or in gymnasiums to good effect, and are ready to ride on the roads when spring arrives. There are training plans and software you can purchase to guide you in indoor training.