No matter local or long distance, every relationship needs to be treated carefully and seriously, focus on both you and your partner's feeling in this relationship. Long-term transgender relationship is unlike other relationship, it is even more harder than any other relationships, however, your careful management may let your relationship in a sustainable and healthy state.
Last but not the least, it's also very important to ask whether you and your partner can accept the long distance before start a relationship, and clear about when the long distance will be ended, it's just like a love promise, and the hope of the long-term relationship. Only in this way, can you have every reason to be hopeful for the future of your long-term transgender relationship with trans girl.