Some exercise is better than nothing.
Studies have shown that accumulating short bouts of exercise can make a difference. For sedentary people, even a few minutes of daily stair climbing - a vigorous but easily accessible form of exercise - can improve cardiovascular health.
In one British study, 22 sedentary university-aged women walked up 199 steps in 2.25 minutes, a “brisk but comfortable” pace which shot their heart rates up to 90 percent of their predicted maximum. They progressed from one ascent per day during the first week to six ascents per day, for a total of 13.5 minutes over the course of a day, during the sixth and seventh weeks. By the end of this modest exercise program, the women were measurably more fit: Heart rate, oxygen uptake and blood lactate levels during climbing were reduced, and their HDL (''good'') cholesterol levels had increased.
Source: Preventive Medicine, 2000; 30, 4, 277-281