Actually, the whole dragon being able to cover vast distances isn't as inaccurate as one might think. Assuming Westeros is comparable in size to Britain, then it wouldn't be unreasonable to assume that the distance between Dragonstone and wherever Jon's coterie was happened to be about 1500km. Assuming Dany covered that distance in one full day's flight (24 hrs), then she'd have been cruising at a comfortable 62.5km/h (38.84mph). Absolutely blistering by Westerosi standards, but by ours, that's a slow stroll to the grocery store.