A few thoughts:
First, everyone always assumes that runners that are successful grinders at shorter distances will instantly have success at longer distances. Sometimes it takes a few races at a longer distance to figure out just how much easier it has to feel for the first half of the race or so in order to actually execute the race properly. Sometimes guys like Mantz are a little too used to getting out on that edge, and you don't want to be out on that edge too soon in a half (or full) marathon. He might be just fine, but we'll see.
Second, Hall had already run the 20k American Record before he set the Half record in Houston, so he had already raced in that ballpark.
Third, I think it is a shame that Hall essentially moved up to the marathon and never looked back after he set the American Record in the Half. I know that is where the money is, and the fame, but I don't think the marathon was actually his best distance. I think Hall's best marathon performance was at the Oly Trials in '07, where they ran a very controlled first 16 miles before Hall absolutely stomped everybody over the last 10. For those that remember the race, it was stunning to watch, not just the pace he was running as he pulled away, but the absolute ease with which he did so. It looked like Hall was almost stumbling over himself as they ran along at around 5-minute pace, and only looked in his element when he started ripping off miles in the 4:30's and 4:40's (in Central Park!). He did that when he wasn't that far removed from that record Half, and the huge negative split played right into that vein of fitness. I wish he had stuck with that more, as opposed to going the pure marathon route he went (I know he "raced" some Halfs after that, but they were almost always within a marathon buildup).