In Bhagavad-gita you will find confirmation of the fact that this Hatha Yoga system is not so easy process as you have been led to believe by various "masters''. To do actual Hatha Yoga one must not live in the city, but he must go to a sacred spot, and completely freed from all attachments and all material desires and disturbances, he must concentrate his mind upon the Lord within his heart. This process is not completed in just six months time, but it extends over many, many lifetimes.
Letter to Martin Malles, 22 July, 1969