Some 10k & halfs can sort of predict a marathon but it's still tough. I know women who will never break 38 in a 10k or 83 in a half (610+ & 625 paces) but can do 3 flat (652 pace). But I also know 30 min + (450 pace!) 10k fellas who can't break 230 (540+ pace) & 36 min guys who can't break 3 (me when I was 35 to 40).