I just caught this thread, so I guess I can chime in, although the majority of my experience is with sprinting. I think I can throw some insight into the mix.

The funny thing about breathing is to remember to do it. Quite a few people have this tendency when pushing themselves that they totally forget to exhale for several seconds at a time, which is terrible. This isn't to encourage hard, rapid breathing but to encourage you to remember to do it. While you won't be in aerobic respiration for long, when it comes to the 200/400 training that's been suggested (it works, its great even for mid-distance runners. The longer you can stay in aerobic respiration, the longer you can pump out some serious force. Even as a sprinter we did a form of interval training [ex. : 100 100 200 100 100 200 break 100 200 100 100 200 200 break 100 100 200 200 100 finish]. We made sure to give our body time to get some rest, but it encourage the development of increase aerobic respiration. The worlds best 400m runner ever, Michael Johnson, stays in aerobic respiration almost his entire 400m supposedly).

About running form: I wouldn't suggest running completely flat footed, but not being "on your toes" is going to be key for distance running. Unless you have some death wish for your calf muscles. In any case, being comfortable in your form is the key, some people run in a strange manner (at least what I call strange anyway). Focus on staying relaxed, don't worry too much about keeping perfect form, but remember the basics: good posture, relaxed arm movement, steady and rhythmic breathing. Everything after that is gravy, really.

If you end up wanting to train for speed I know I can help in that department. Unfortunately anything over 800m and I lose a bit of knowledge and tend towards generalities.