The book on which the film is based is not credible.

Rating: Null/10 (6 April 2011)

In 1958, Dr. Hugh Richardson, a civil servant and scholar who was one of the world's leading authorities on Tibet, wrote a review of "The Long Walk" for The Alpine Journal (UK). Based on his knowledge of the geography of the region, including India, Tibet, Sikkim, Bhutan, and the Gobi, and of the cultures and habits of the peoples there, he concludes that the journey did not happen. Aside from the claim of going for 12 days without water, the descriptions of places and of people do not jibe with the reality on the ground during the time of the walk.
Some examples from his review: "Apart from Lhasa - which was 'bypassed' - there are no place names; there is also rather scanty detail of scenery, local customs, etc, and what there is is quite uncharacteristic of Tibet." "The abundant wildlife on the greater part of the route through Tibet is unnoticed." "Mr. Rawicz tells of a 'succession of villages' in the deserts of North Tibet and of great stretches of uninhabited country in South Tibet and in the Indian Himalayas which are known to be populated. That must create grave doubts."
The full review could be obtained from The Alpine Club in London.