Will 20 minutes of exercise everyday for half a year make any difference?

On September 10, 2011 I will start an experiment exercising only 20 minutes a day for half a year. The 20 minutes will include warm up (if necessary) and stretching.

Details
I will exercise the 20 minutes anytime during the day, when it fits me. Sometimes it will be in the morning and sometimes in the evening. The type of exercise will vary but mostly I will run at moderate pace. Push ups, sit ups and other type of strengthening activities will also be part of my exercise. In other words I won't have any specific rules on what I should train within the 20 minutes. In order to make it as pleasant as possible I don't want to be too rigid on how I train.

Food
I will not intentionally change the way I eat, however the food intake might change as a result of the exercise. Basically I will eat whatever I feel for, as I'm doing today.

Current fitness
I'm pretty unfit at this moment. I'm weighing around 92 kg and I should probably by weighing around 77-80 kg.

Measuring results
* start - I will run 10 km on September 9, 2011 in order to have a benchmark. I will measure time and heart rate as well as the trail.
* everyday - I will measure time, distance, trail, heart rate
* once a week - weight, muscle mass, fat mass, measure abdomen, chest and upper arms
* The experiment ends on March 10, 2012. On March 11 I will run 10 km. Once again I will measure time and heart rate in order to compare to the run from September 9, 2011.

Purpose of the experiment
To find out what difference a 20 minutes exercise will do. Many people says it won't make any significant difference while other says that exercising only a small period of time will make difference. We will see if it makes any difference for me!