@LateRunnerPhil: How do you come to these conclusions? Is this your own experience or are you a coach?
My own experience is that the fitter I get, the more I burn. Only marathoners seem to get better at burning fat, I just burn more sugar. I run 28 low for 10km and 13:3x for 5k. My training leaves me absolutely depleted if I don't eat enough. I know many other runners who run between 13:20 and 14:00 for 5km and they almost all eat like me. 3500+ kcals / day. The ones who don't are extremely skinny and often have stress fractures.
For reference I'm 183cm 70kg.