There isn't necessarily a perfect correlation between 10K and marathon splits. Look at Dick Beardsley. IIRC he never broke 29 for 10k, and on paper, there is no way he should've run sub 2:09 on such a "weak" time as that. But nevertheless he did.
People that obsess over times correlating to other times don't fully understand that some bodies react differently to training for different distances.
Just because you're a 1:45 800m runner does not mean you should be a sub 8 3000m guy.
Running the 10k in 28 minutes does not mean your half/full marathon will be a sub 2:20.
Not many Athletes are like Rui Silva, running a 3:29 PB 1500m and then turning around and becoming a high level half marathon guy.