Dylan Thomas's pedigree looks to me like that of a 10f performer. It's not impossible he could get 12f, but it looks pretty unlikely.
His sire, Danehill, we know to be neutral in terms of the stamina traits he imparts to his offspring, and his broodmare sire, Diesis, can also get good winners over a variety of distances, although not to same marked extent as Danehill.
There's not much in the bottom half of the pedigree which suggests 12f is the distance to be passed on to Danehill to work his magic on, there's a mixture of speedy types and middle distance animals, you'd "sort of average it out" at around 10f.