I think the only thing she MIGHT not have down is access to food and water once she starts getting further west. If she's mentally okay with being away from home for such a long period of time, there really isn't anything physically different about that, than about being out for a week or two. After the first four days your body's in shape for the mileage, particularly at her age. But she may not be prepared for areas where it could be 50 miles or more between convenient stores.
I was even more clueless, and about the same age, when I set out on my first solo tour. But I was never anywhere that I couldn't buy food or pay for a campsite that supplied water.
Speed comes from what you put behind you. - Judi Ketteler