There is a lot of evidence that the human body can survive for much, much longer than 72 hours without food:

http://www.scientificamerican.com/article/how-long-can-a-per...

http://www.bbc.com/news/magazine-17095605

The first article mentions someone going for 73 days without food, the second 66 days, and many others for shorter periods (but still on the order of multiple tens of days).

Some examples:

"In the 1940s, Mahatma Gandhi survived 21 days on sips of water during a display of civil disobedience."

"Dr Mike Stroud, senior lecturer of Medicine and Nutrition at Southampton University ... who accompanied veteran British adventurer Sir Ranulph Fiennes across the Antarctic, said it was possible to survive 60 days without food."

"In 1981, Republican prisoner Bobby Sands died in Northern Ireland's Maze prison after a hunger strike lasting 66 days."

Of course many factors may influence how long you can go without food, such as the amount of physical activity you engage in. If you're running marathons every day, you're naturally not going to be able to go without food as long as someone who's immobile the whole time, etc.