@Doomer
I think she's trying to avoid killing the messenger. I'm guessing if the messenger makes a normal pace, then the messenger will probably not be punished. Also the messenger and prince can assume that they are constantly behind the messenger, not in front, making things a bit easier to move around if they aren't expecting her to be in front. Buuuuut there are loop holes around this.
Why not make a potion that makes her look a year older than she looks now?
The look of a 16 yo is not that different from a 15 yo but it is the same as an adult of 18-22 anyway...