Lagat is obviously the best 40+ distance runner in the history of the world.
1) Geb ran 1:01:09 when he was 40 y.o. Lagat will be 42 y.o. when he attempts the half marathon master record. That extra 2 years can make a difference.
2) The half marathon distance is pretty long for Lagat and his best half marathon is only 1:02:33 (although I think that he may only have enough the distance once). He would have to PB by more than 1 minute.
On the other hand, he ran a 27:49 10,000m on the track a few months ago, so his endurance is good at this moment.