Hi, I'm also a girl and i think you should really go for it. Muay Thai is not limited to just guys that i can assure you. I have trained Muay Thai myself. It's all about your attitude and what you are looking for for yourself. Muay Thai will definitely help get you in shape. It requires a lot of basic exercises like running, rope jumping and also a lot of disciplines.
So don't doubt about the sex and shape. Go have fun!
I've been in Muay Thai business for more than 5 years with MuayThaiArt
and many more. Let me know if you need my help in the field!