Think of taming a thestral a lot like taming a wild horse; it's going to take some time but she can do it if she's patient and works on building its trust. It would take about 6 months to make one of them "tame" enough to be called a pet, and after that point it could be trained for riding, which could take four to six months. A baby one would naturally be easier to tame, but it would need more time to ride since it has to grow up.

Getting a thestral from either city into Discedo would prove difficult, considering she would have to tame the animal first in order to get it from one place to the other easily. Taking it by train would be impossible, so she'd have to travel on foot/horseback to Discedo. She COULD do it of course, but it would take time.