The Hour record is 21,101m, set by Mexican Arturo Barrios (former WR holder in the 10k). That distance happens to be extremely close to the 1/2 marathon distance, which I calc as 21,097.5 meters (correct me if I am wrong). So.....clearly Geb mainly wants the Barrios HOUR record (and Barrios's 20,000m track record set en route), BUT.....
Wouldn't it also very interesting to see if he can run the fastest unofficial 1/2 marathon too?? In order to do so, he would need to beat Barrios' distance covered with about 90 seconds to spare on his hour time limit, and then continue on for the hour (and in fact could ease waaay up at that point). THAT would get him the official One Hour record, the 20,000m track record, AND.....the unofficial fastest 1/2 marathon ever run (beating Wanjiru's 58:35 record, a record Geb I am sure was not happy to lose).
Geb may not be planning on trying this, but on the other hand, we all know he is very driven to set as many records as he can, official and unofficial. Even though it would unofficial, I would think he might be in the back of his mind thinking about that "fastest 13.1 miles ever covered" concept. Can he do it? The 1/2 marathon on the track might be his best of distance of all right now! Think about it: he's been marathon training for awhile, but he doesn't seem to be a perfect fit for that distance, the track seems to be his best surface, and his recent sub 27:00 10k shows he still has the speed. I say he comes damn close!