I recently rented the PBS NOVA special "Marathon Challenge". I absolutely loved it. It takes us through the lives of 13 couch potatoes as they try to run the Boston Marathon during after training for 9 months. Rather than making it painful to watch, NOVA had trainers and medical staff to minimize the likelihood of injuries.
These couch potatoes started from almost nothing--walking at the absolute most. They were checked out by doctors and cleared to run. From there, we saw the struggles of just running 2 miles all the way through the last training run of 20 miles. They had a coach as well as Uta Pippig, a previous Boston Marathon winner.
What I really enjoyed about this is how medical-ly it was. Being PBS, they were very educational. It talked about how the muscle fibers worked in the legs and how endurance running helps blood flow, etc. It was very interesting for something like me who has been running for a couple of years. It certainly puts the why into how we become fit.
Finally, it was absolutely heart warming to see people start from nothing and then finish a marathon (though not all of them did). It's similar to the feeling I get when I watched the Olympic marathons. The big difference is that I knew the Olympians were going to finish it; I wasn't so sure about the participants on the documentary.
Though I rented it, it's also available to watch online. If you watch it, let me know what you think.