Whether it is yoga, walking, running, swimming, or dancing. What ever it is that makes you feel happy, just do it! A few minutes a day can go a long way. You don't have to be an athlete to improve your health.
But, how long is truly enough? The answer depends on the kind of life style you have and on your body. If you have a job that requires you to sit for many hours at a time, try getting up at least once every hour. Walk around for 10-15 minutes. This will improve your circulation and thus your blood pressure.
If you are normally active 20 minutes a day will be enough to keep all your systems running smoothly.
The point here is to move. Our bodies were made to do so. So...let's do it!