But the Riverlands were teeming with bandits, and the Lannister armies were there as well. And even if they managed to pass through the Riverlands unmolested, Moat Cailin could block the Vale's full force with only a small garrison. And by that time, Sansa Stark and her half-brother would be either sitting in Winterfell, and he'd be late to get any credit and favours, or they would have already died trying. They would not be able to set up a proper supply line, and foraging in late autumn was not reliable by any metric. The Riverlands had probably been stripped bare of food by bandits, Lannisters and Freys, and the North had been scoured by Ironborn. Not to mention that Sansa could also have died in the harsh northern weather or decided to escape into Essos.