Yes, you can! The catch is that you have to workout as hard as you can.
In 2006, researchers at Ontario's McMaster University took 16 men and divided them into two groups. One group biked at a moderate pace for up to two hours. The other group did six, 30 second full throttle bicycle sprints. These workouts went on for two weeks. After this time period, the researchers took tissue sample from the study's participants. Both groups demonstrated similar changes in the tissue samples - demonstrating improved muscular fitness.
Keep in mind that one group exercised for hours over the two week period and the other exercise group exercised for less than half an hour!
If pressed for time, working out for a shorter time interval but with increased intensity may be a good option to increase fitness. However, it is still important to get regular cardiovascular exercise which is exercising for 30 minutes or more at a moderate pace.
Mix it up! Perhaps doing moderate cardiovascular workouts on certain days and then doing hard sprints for short intervals on a stationary bike, treadmill or jumping rope on the days that you just don't have time for a workout.
Information for this article came from an article in January's 2013, OUTSIDE magazine by Christopher Solomon.