hey bro,
you'll love jiu jitsu. I've taught friends who were in "good shape" and watched them huff and puff and sometimes puke. Start slow, be careful for injuries. Don't get too egotistical or cocky... you'll pay for it eventually.
one of the big questions people ask me is GI or no GI?
I think its best to start wearing a GI, get your balance and what not. Then in a few months start taking a class or two per week without the GI. (most academies have both) You'll find its sometimes an even harder workout since it tends to move faster and its harder to finish when you're slipping and slidin all over the mat. After you get used to that, maybe mess around with a little Muay Thai and practice take downs - combine that all to start a little Vale Tudo/No Rules training. Thats when you'll be feeling the burn. And you'll be a monster by then.
most imortant... have fun with it.