as above I have done zen do kai, mauy tai, a little kung fu, and taekwon do..
TKD is very focused on kicks , so will be great for flexability, and general athletisism. as for fitness i have always found that you need to extend this to outside the dojo, as in class just doesnt give you what you need, ( I would run 5km 4 times a week, and 2 hours of 3minute bag work 3 times a week,) when I was right into it. a huge 56kg lol,
I have never made black belt in anything but rather one belt under at everything...my technique was always up there and beyond, but I hated carta, and one step etc,
like you I just wanted it for fitness etc not for fighting..