Let me just say that while I am not out of shape, I am well past the age where I would personally wear this thing as a dress! But perhaps I'm just not adventurous enough...
I bought it to wear as a tank top, and with a little bit of "scrunching it up" around the waist/hips, it'll work perfectly for me as a long-ish tank. Could also see it working with leggings, but I don't want to melt...
I'll also say that I generally wear a small or medium depending on the brand, but I sized up to a LARGE based on reviews and that was definitely the right thing to do.