Visiting south of Madagascar

Hello,

My name is Christophe and I am Mauritian. I am coming as a tourist in Madagascar during the first 2 weeks of September (from 3 to 16) and I will visit the south of the island starting from Antananarivo. I have check some guide book and the south is the part of the island which attracts me the most especially with the train ride.
I would be grateful if you could advise me on an itinerary to the south along the RN7, which can hold itself within the 2 weeks (knowing that I would like to be two days before my departure on Tana and that I will travel by taxi)
Thank you for your help and do not hesitate to give me any other advice. I take everything that can help me prepare my holidays ;-)