Well, here's my story.
I've been a lacto-vegetarian for twenty-five years. For much of that time, my diet has been essentially vegan (no diary products), and has generally been very strict (e.g., very low sodium, very low fat) even by vegan standards.
I became a vegetarian when I was nineteen. At the time, I was a junior varsity runner at a Division III school (M.I.T.). My best time for any track race was slightly under ten minutes for two miles (which I ran on perhaps the fastest indoor track in the country). I was in my fourth year of competitive running.
Over the ensuing years, I improved to the level of a sub-30 10k runner (at the age of 24) and sub-2:20 marathoner (at the age of 30).
I have always avoided making claims of athletic benefits from my diet. As far as I can tell, my improvement in running has come from hard training, not through dietary manipulation.
At the same time, I will admit to having gotten a bit irritated at times when people would suggest that I would run faster and get injured less often if I had a steak once in a while. I got injured a lot when I was a slow, low-mileage meat-eater, and I got injured a lot when I was a considerably faster, high-mileage vegetarian.
I have no idea what diet is optimal for a distance runner. If I could do things over, I would probably be more careful about supplementing my diet with iron, calcium, various anti-oxidants, and a few other nutrients (including fatty acids). I would probably also increase my protein intake somewhat, particularly during periods of heavy or intense training. Beyond that, I don't know.
I believe that, if there is one major flaw in the diets of most American runners, it's that they consume too many calories for the amount of energy that they expend. (I realize that, for some runners, the opposite is true, and I don't mean to belittle the legitimate concerns that have been expressed about those individuals.)