Too much is being expected from yoga which is nothing but a mechanism of keeping the organs and muscles of the body in a proper functioning mode. To say that extra strength of physical and mental type will be obtained is not right. A feeling of well being will occur. 

Sorry. You are under the wrong impression that Yoga give mere feeling of well being. The entire world has recognized Yoga as a holistic therapy which is capable of keeping both physical and mental  strength. Here is the scientific proof

http://www.bloomberg.com/news/articles/2013-11-22/harvard-yoga-scientists-find-proof-of-meditation-benefit

Now you are saying 'keeping strength' earlier you said it gave strength. Which is true?

Over a period Yoga gives strength . The good results of Yoga will not start from the day one. One starts yoga with an aim to keep strength. Over a period of adhering to it, Yoga Gives you mental and physical strength.

I do yoga on and off and personally I feel that it is good for keeping your muscles and limbs flexible and also helps those who have breathing problems...But brisk walking for 40-50 minutes a day is the best for strengthening your calf muscles and also keep your weight in check , body toned and in fact it is a complete exercise form, that increases strength and your energy levels..

Physical exercise or Yoga it may be anything to suit the individual. Important thing is ACTIVITY..

Physical activity especially Walking and jogging are best for losing weight and in the process your energy levels too go up...You cannot depend only on Yoga to lose weight and increase your energy levels , you also need physical activity.Whereas jogging and brisk walking is a complete exercise pattern that takes care of everything  - increases your energy levels, helps you lose weight and maintain it !

I have been doing these Yoga Asanas for the last 25 years. And i do not have a single cm of fat in my body. And for weight, my weight remained frozen at 67 Kgs for the last 15 years. My Height is 5 ft 11 inches. When it comes to energy there is no dearth of it. I work daily for 8 -11 hours without tiredness and fatigue.  I work not only for this site, but I work for two local Publishers. Dependence on physical exercise alone is not advisable as it gives you only physical strength leaving mental sturdiness.

 Above all yoga boosts your self confidence which further leads to self - reliance.

For me Yoga is a miraculous holistic therapy.

http://www.prevention.com/fitness/top-yoga-poses-weight-loss

Thats exactly what I am saying that yoga by itself is not enough to sustain energy and weight loss.maintenance.It has to be combined with physical exercise to do that.Either daily routine or jogging, brisk walking ...I have experienced it myself , walking and jogging really boosts up your energy levels and helps in weight loss much more than just yoga..

I did not say yoga should not be clubbed with physical exercise. The content of my above post is, when Yoga itself is a holistic therapy embracing both your physical well being and mental well being, there is no necessity for a separate regimen for physical exercise. If one wants to combine both they can. But in my opinion there is no necessity.