From what I can see, even if your 10k half record is good

It seems that people who can't finish a full marathon struggle because they lack long-distance endurance.
A female running influencer I enjoy watching, Miles, has great 10k records and a great physique (height minus weight ratio of 112),
everything is perfect, but seeing her full marathon records are low,
I looked at her training logs and noticed she hasn't run anything over 20km..