I've heard some Ethiopian or Kenyan refer to races up to 10k as middle distance before, also (I think it may actually have been Geb)...if you consider how much longer the marathon (and even the half) are than the 10k, it makes a little more sense...a 10k is only 5.2 more miles than a mile, but a marathon is 20 miles further than a 10k...
these distances are PURE aerobic, and you can succeed at them with relatively little track speed...once you've got guys that might do one set of events without doing any that are "under" them, you can classify that as another group of distances
it may be common to see a 10k/marathon specialist, but it's also common to see a 1500/5k specialist...