The book "Born to Run" discusses the theory that humans evolved as long-distance runners. As far as the science goes, I feel it is a work in progress. As time goes by and more studies are done, we may have more evidence one way or another about the role of running in human evolution.
Until then, you can always look at the practical experience of people to gain insight. There were a few things about distance running in the book that I didn't know. First, distance running is the number one participation sport worldwide. I think this tells you something in itself. In the book this is explained by the statement, "recreation has its reasons." In other words, there is some biological drive behind millions of people making the decision to run.
Second, the book talks about how much of the running done for persistence hunts was at an aerobic pace. In one section of the book, it talks about a 10-minute mile pace on the trails as a pace that will allow humans to keep up with and outlast game. It is interesting that this moderate paced running is a central part of many successful running systems, like Arthur Lydiard's. Lydiard prescribed runs of up to two hours to build an aerobic base, and unless you're an elite athlete, you will end up running this at a moderate pace.
I did some experimenting with different paces at the track tonight. I went faster and slower to see what paces felt comfortable and could be sustained over long periods. I wasn't timing it exactly, but it was fairly close to 10-minute miles (barefoot, on grass). There are certain paces in running that you can lock in and it feels like you can run forever. If you are on a persistence hunt and you don't know how long it will last, this is the type of pace you want. If running was central to human evolution, my bet is that these moderate, Zen-like paces were part of it.
There is also the nutrition aspect. If you lock in these moderate paces, there is no big drain on glucose, and you won't need more carbs to sustain the effort. Mark Sisson has talked about this in his piece on Chronic Cardio.
I think the key is accepting these moderate paces as being okay. Everyone wants to race and get faster, but to do so you many people end up doing long runs at higher intensities. It is this combination of intensity and distance that can lead to trouble.
