Here's another perspective from an active.com article:
http://www.active.com/running/Articles/The-Great-Gebrselassie-Breaks-2_04_-Is-Sub-2_00-on-the-Horizon_.htm?act=EMC-Active&Vehicle=Running&Date=10_01_08&Edition=1&Sections=Articles&Creative=The-Great-Gebrselassie-Breaks-2_04_-Is-Sub-2_00-on-the-Horizon_&TextName=Read_The_Full_Article_On_Active&ArtText=Txt&Placement=1&Day=ThuKey quote from the article:
"Now that 2:04 has been broken, the record has been chopped halfway to 2:00, from 2:08. Will it be another 23 years until 1:59 is reached? Or will the next four minutes take longer to erase?
No one can say for sure, of course, but the idea is not as crazy is it once seemed. Gebrselassie's countryman Kenenisa Bekele is the current world record holder for 10 kilometers, and has been selected by many as a prospective marathon record holder—perhaps even the man with the pedigree to approach two hours.
His 26:17 for 10K works out to 4:14 per mile for the 6.2 miles. In order to run a 1:59 marathon he would have to average 4:34 per mile. That's 20 seconds per mile slower for a little more than four times the distance. Does that sound doable?
Tanui himself has gone on record as saying he thinks sub-two is possible, based, he says, upon how he felt after running 59 minutes for a half marathon. It's easy to say that after you stop halfway! You can look at the numbers any way you like, but in the end anything will be nothing more than an educated guess."