Goes to show how much I know of Westeros geography, lol. So, yeah, even if they managed to stave off death for about 5 days (kinda unlikely given that they were in a frozen shithole), and assuming the bird needed at least a full 24 hours of rest (in total) to get to Dany, and one day for Dany to travel, then she'd be clipping along just over 150km/h (~100mph). Still not infeasible, given we know exactly nothing about how fast dragons can really go when they're at full tilt, but the party living nearly a week on whatever rations they had in the middle of a frozen lake with nothing but their own body heat does kinda beggar beleif.