However, your level of fitness will depend on the following:
The levels of fitness that you achieve through yoga will not only depend on the style of yoga you choose but also on the frequency with which you practice as well as the length of your practice. If you practice 15 minutes of gentle yoga stretches three or four times a week, you will also need to do some other form of exercise to stay fit. You might want to pair your yoga practice with walking, biking, running, dancing, elliptical machines etc... or increase your yoga time, style or frequency. Studies done on yoga have included more than an hour of practice two to four days a week. The yoga sessions included breath work and meditation in addition to typical yoga poses. Finally, the asanas used in these studies included not just aerobically challenging sequences, like Sun Salutations, but also many strengthening poses.

In my humble opinion, the best exercise is the one you love, the one you will do regularly and stick with. So, if you are happy with your yoga practice and don't enjoy other activities then by all means continue to practice and enjoy it! However, if you enjoy running, walking, hiking, biking, dancing, or any other form of cardiovascular training then add that into the mix! Yoga is a philosophy that promotes self-expression and personal choice. After seven years of practicing yoga solely, I felt the need to move my body in different ways. I began to take Salsa and Tango dance classes. In the last couple of years in addition to having a regular yoga practice I have continued to dance, enjoy biking, elliptical machines and light weight training.

As well as fitness, yoga offers many other advantages. It improves your overall health, reduces your level of stress, combats insomnia, increases energy, and elevates your mood and general outlook, which can improve many other areas of your life.

References: Yoga Journal