I'm a teenage girl and if you want to get fit and toughen yourself up well then this is the sport for you. Its so much fun and your building your body up without even realising. You use a lot of energy also and for different positions different fitness levels are needed and also some people are bigger than others because the big ones tackle more while the small fast ones run to score tries :-)
Much more important than fitness in rugby is heart. Just go along and always do your best, your team will respect that. Stop procastinating, go and sign up now.
Rugby is the running in soccer, combined with the tackling in football. It will put you in good shape.