For metabolic effects, the most effective and time efficient form of exercise is HIIT; high-intensity interval training. A typical HIIT workout alternates periods of short, intense anaerobic exercise with less intense recovery periods. The intense exercise periods are performed at 80-95% of your maximum heart rate.
There are four factors that make up a HIIT workout; the duration, frequency, intensity and length of recovery intervals. The relationship between intense work and recovery is important; studies use a specific ratio in order to improve the energy systems of the body. For example a 1:1 ratio workout would be 4 minutes of intense exercise followed by a 4 minute recovery, other workouts might be 30 seconds of maximum intensity exercise followed by 4 minutes of recovery; repeating this 3-5 times.
You can do a HIIT workout with any type of activity such as running, swimming, cycling, walking and even strength training. Here is what I do: I run on a treadmill for 2 minutes at my maximum intensity (HR at 220 - age x 80%) and then walk slowly for 30 seconds while I catch my breath; repeating this 5-10 times. HIIT can be easily modified depending on your fitness level and health status.
HIIT has been shown to improve blood pressure, cardiovascular health and insulin sensitivity. Studies show that HIIT training significantly lowers insulin resistance compared to other forms of training, leads to decreased fasting blood glucose levels and weight loss. It also helps to optimize heart and lung fitness and has many additional health benefits.
Here is a link to a page that shows various HIIT workouts you can try; or keep it simple and practice HIIT on a treadmill or elliptical: http://darebee.com/programs/30-days-of-hiit.html