It depends on what kind of yoga you are doing and what your expectations are.
Will it help you become more flexible? Yes. Will it help you improve muscle stamina and strength? Yes. Will it help you develop better posture, help you become more aware of body mechanics? Yes. Will it help you develop a more centered attitude, increase your mind-body connection, help you deal with stress, etc. Yes, but I think those things would be harder with a video than a live class.
Is it equivalent to cardio exercise or weight training? No. Some yoga is done at a very fast, intense pace and can function like cardio training, but that's not really the intention. It also cannot take the place of strength training with weights. There ARE plenty of people who stay in shape by doing yoga alone, but again it's not going to work your body the same way weight training will.
I love yoga, it's added so much to my life in a physical and mental way that my other types of exercise did not offer. It can take a while to really get into it and absorb the principles in a meaningful way, but it's worth it.
P.S. It will be really worth your while to seek out a class. Yoga is all about nuance and precise positioning, and it's incredibly helpful to have a teacher there with you to help you get the most out of each pose and really get the full effect. You might need modifications or alignment help, and a video can't give that to you. Still, doing a video is better than nothing.
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