Although I've taught Qi Gong to thousands of people around the world over the past decade, I'd like to introduce you someone else whose work I consider authentic and powerful...
Shifu Yan Lei, a 34th generation SHAOLIN master from China, now living and teaching in the U.K.
As you would expect, Shifu teaches the classical Shaolin Qi Gong forms, such as Muscle-Tendon Changing, the related 8 Pieces of Brocade, Bone Marrow Cleansing and more.
Some would dismiss them as "hard" or "external" Qi Gong forms, but I beg to differ - these forms are excellent for building health, fitness and do contribute to longevity as well. Moreover, the Taoists, who specialized in longevity, respected and adopted these methods.
Keep on training :-)