As you have noted, the 10K seems to have fallen off some over the past decade or so. These things seem to go in cycles. As the half-marathon popularity has skyrocketed, the growth rate for the marathon has diminished significantly. The half-marathon certainly will not be going away anytime soon. The 5K seems about as popular as ever — though I still do not get having 5K, 10K, XK race distances yet split markers only in increments of miles, it is like producing yard sticks with only centimeter marks. Who knows what will come next for the participatory masses, the eliptigo marathon?