This is advise, I would give my own daughter and she will be 16 in a few weeks:
"I am proud of you that you want want to maintain a healthy lifestyle by taking up running. Running takes commitment, consistency and a love for the pavement. I understand you have been only doing it for a few weeks but change does not come easy and it does take "more than a few weeks" for you to see improvements in strength, endurance, as well a physical and mental changes. Please do not focus on the cellulite, rather focus on improving your state of mind on how you see your body and more importantly on your self esteem. When I look at you, I see a beautiful young lady full of love for life, a creative person full of kindness and empathy as well as an intelligent and hard worker. Let's work together to eat whole natural foods and as well as enjoy and make some of our favorites dishes and desserts. Your cellulite concerns shouldn't occupy that beautiful mind of yours at this age! Let's go get you some new running clothes. And....... I love you, but you know that already!"