DD is going on a school ski trip during Feb half-term. She has never done this before, and we have realised that she needs to work on her fitness. She's a typical 13year old who likes lounging around in her room and is not very sporty. She bikes a little, but stamina is not great, and she could probably work on her leg strength too. What should she be doing beforehand to make sure that she is fit enough to really enjoy herself?