I want to start off saying that I am not an elite athlete of any sort, and running is definitely my weak point. That being said, I feel there need to be points made on the other side of an ongoing discussion.
There has been much debate lately about the length of the more serious obstacle races, and that they favor runners over more well-rounded athletes (in particular, Crossfiters). My feeling is that if you can’t effectively cover distance past 10k, you are not really a well-rounded athlete.
Crossfit is a great attempt at what is probably an impossible goal, a single universal training system, a system with no weaknesses. The idea was to specialize in not specializing, to create a single workout program that could adapt to any endeavor. The problem is that by focusing on short, intense workouts, they specialized. Short intense workouts are great… if you are training for a short intense task. This is why great 5k runners rarely make great marathoners, they program themselves to burn up too much too quickly.
There are great athletes out there combining crossfit tactics with tactics more suitable to distance work, and in my mind those are the ones who CAN handle any endeavor that is thrown at them.
I tend to think of military training in conjunction with all-around ability training. As H.W. McBride said, a rifleman not only has to be trained to do everything, he often has to actually do it. All elite military training that I have seen involves a lot of long distance running and rucking.
Speaking for us non-elite runners, the distance contributes greatly to the challenge. You can fake your way through a 5k or 10k (I’ve done it). It is difficult to fake your way through a half or full marathon, even without obstacles. There are a great many of us who don’t care about beating someone else’s time, but only in beating the course. No matter how obstacle-dense it is, the longer races push us harder, force us to dig deeper and become better. For those of us who don’t end up on a podium, that is the whole point.