Yoga is not the be all and end all of exercise. Yoga is a cardio vascular exercise, which can be done in other forms. Brisk walking is a good way to start, don't start with four kilometers a day, try to walk as much you can and slowly increase the distance. If you force yourself from the word go you may give up on it in no time. Jogging is also wonderful, but make sure that your doctor approves of it if you have any health issues.
Learn breathing exercises which are good for your lungs and one of the benefits of Yoga. There are many good videos on youtube to help you with it. Learn to relax , take deep breaths and let out stress. Taking deep breaths helps you calm down when you are tense, One of the benefits of doing yoga is to reduce stress.
Try meditation, take a scripture from your holy books which grabs your attention and keep thinking about it, reciting it, expounding on it all help. However, this is primarily a mental exercise..
Doing all these will help you reap all the benefits of yoga.