One of the first things any serious practitioner of Jiu-Jitsu needs to understand is that ego is the first and the biggest hurdle to overcome. Learn to take each tapout as a learning experience. The people who end up doing the best are those that can lose with a smile. Those people enjoy the art purely for what it is rather then for what that want to make of it.

Yes, rolling is an aerobically intense activity. But the best way to build up cardio for this is by rolling.  Jiu Jitsu requires balance and mat awareness. The movements are very unpredictable and require muscular power and endurance, flexibility, balance, and cardio.

Don’t get discouraged. On your first day, everyone is going to have more experience than you do.  Ask questions of your instructor and the higher level belts. Rely on technique, not strength. Know when to tap out, and don’t be afraid or ashamed to do so. Try to escape, of course. But if you can’t, tap.

There is much in BJJ that is counter-intuitive. Things like creating space, reaching, framing, posture… they all take time, practice and solid instruction to understand. Pay attention to lessons. Never stop asking questions. Carefully review how you get cornered or setup for subs and passes. Never get frustrated, because the mind shuts down when frustrated and you will miss the finer details that make all the difference.

Finally… breathe. Slow, measured. Don’t rush. Don’t burst. Don’t strain. You don’t rush into it at full speed and try to muscle people. Take it easy. BJJ is a flow, not a fight. Train at a reasonable pace and the technique will become muscle memory before you know it. 

It's impossible to get in Jiu Jitsu shape without practicing Jiu Jitsu. While it certainly doesn't hurt to get in better shape beforehand, it isn't necessary. Dive right in and start BJJ. Your fitness and skill level will improve through attending class regularly. 

If you're holding off because you think you could do with a bit of preparation first, you may never get around to actually training.