My best advice .... especially for a teenage girl will be to see a sports nutritionist to get a proper diet sorted for her height/weight/ training needs.. and also get her to take iron supplements and possibly magnesium..shouuld fatigue start to kick in. Endurance training is taxing on your body and takes a while to start building.
Golden rule - be sure to eat low GI complex carbs and protein. You need slow release energy so you don't spike - whilst protein for muscle building/ repair!
this topic almost got away, its been the only interesting thing on here for a while so we should keep it going.
i'd like to give some good input but my 8-10km a day is probably cancelled out with a six pack and a 1/2 bottle of wine a nite , but i work in retail and customers are to demanding to stay sober