Trainers know that the most efficient way to exercise is to make sure that you incorporate a mix of strength training, interval training and cardio/aerobic activity.
Strength: Even 20 minutes a day, two days a week of free weights, machines at the gym, or resistance workouts will help you get stronger.
Interval: Switch between running and walking every two minutes during your workout. It will keep you from zoning out and make your time more productive.
Cardio: Walk, run, dance, bike, cross-country ski...if you can get up to 60 minutes of day of some kind of aerobic activity, you'll be golden.