I ran two half marathons and I can tell you that it takes no special training. Anyone who can walk at a relaxed pace for a couple of hours can pull off a half marathon. If you want to train for it, train your willpower. That's all you need. Also, do a 10-15 minute sprint session twice a week and walk everywhere you can, if you want to do well.
Forget the long runs, forget Jillian Michaels (who, by the way, is a sadist who is paid to torture fat people), forget boot camp. Forget everything but walking and a couple of all-out sprint sessions per week. The sprints are where your cardiovascular strength and endurance will come from.
The half marathon itself is not Primal, so let's toss aside any illusions to that effect. I can think of only two reasons a prehistoric human might have needed to run 22 kilometers nonstop: he was trying to run down an incredibly strong prey animal, or he was trying to escape from a hunting party of people who were pissed off at him. Can you think of others? In either case, the lengthy run would have been unplanned.
The half marathon is a modern exercise that people use to create some kind of feeling of achievement for themselves. It has no health benefits that eating properly and not sitting on your ass all day don't have, and in fact has quite a few health risks.
You lousy kids! Get off my savannah!