If by yoga, you mean asanas, then definitely it is not enough to keep you fit. Fitness depends as much on the state of mind as on the state of the body. Thus a well balanced diet, healthy lifestyle, an appropriate yoga routine (asana, pranayama, meditation) and using the knowledge given in the courses can help keep you fit. And doing all these things are very natural and effortless for all of us. That’s why in the yogasutras it is said, “Prayatna shaithilya anantha samapattibhyam." Letting go of the effort and merging with the Divine presence are the two aspects of asana practice. To experience yoga, it is best to learn with a trained teacher and experience the totality of yoga.