So when Susannah (and Mia in Susannah's body) arrived in New York Mia could grow and attach her own legs to Susannah's body.
But why couldn't she do it in the Calla, in Roland's world? It would have been much much easier to flee. New York wasn't her world. Nor was Roland's. So what's the difference?
So why didn't she use her legs after the battle in Calla?