One of the most famous dragon lore masters observed that dragons can go "half way round the world in a week". That averages out to roughly 75MPH at typical altitudes (dragons rarely fly much above a mile without a compelling reason). Dragons can fly much faster but this lower speed lets them make very long flights without being exhausted when they arrive. Tired and ravenous but not exhausted. Another lore master shortly after Gerald's time estimated that dragons could fly "up to 200MPH in short bursts, a speed no man can even dream of achieving". (If only he knew!) The problems are basically these.
Flying dragonback would have been painful- goggles were not yet common in the 1500s. At 75MPH your eyes dry out very quickly.
The northern latitudes are not known for their warmth; much of the year even 75MPH (nevermind 200) would have required a lot of clothes. This makes it difficult to move much and adds wind resistance. And that increases the likelihood of being blown off.
Even in minimal clothing it would be difficult to stay on a dragon at 50MPH, nevermind 200. While it's doable at low speeds it's not easy. No dragon is going to allow even a harness, much less a saddle.
So if you ever hope to ride with a dragon, prepare to be carried in a claw or, if it's cold and you're "lucky" perhaps in his or her mouth. Although if a dragon were to offer you a safe ride on its back, you can bet it would fly in such a way that you would stay on. The laws that govern our interactions would guarantee that.
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