As a fan I much prefer a 10K on a track. But I think the sub 2 hour marathon will go down first...
Without looking it up, seems like it has been about 10 years between the last two 10K world records, both times reducing it by ~5 seconds each time. Most distance runners make little money, and the big bucks are in the marathon... free market theory here :)
So when a distance runner gets under 27 minutes they start looking at 1/2 and full marathons as a source of income, especially the east Africans.
I actually predicted a few years ago that Samuel Wanjiru could break 2 hours. Remember he broke the 1/2 Marathon world record as a junior (he was 18), is the only Kenyan to win an Olympic marathon, and most marathoners hit their peak around 30 or so. His best was 2:03 and change, if I remember correctly. I think he was the highest paid distance runner in 2010 (around $600K)...
... but alas, at the young age 24 his wife found him bed with another woman and somehow he fell off the balcony to his death a few minutes later. There has got to be a moral in that story :)