Define what shape you need to be in to do Jiu Jitsu. The only way you’ll reach the shape you need to do BJJ is by doing BJJ. Simple as that!
Jiu Jitsu is the perfect way to get in shape by itself. It is a great full body cardio workout, and is much more engaging than the gym. Why would you get in shape to get in shape?
The worry that you’ll be the person who can’t keep up is all in your head. Walk into any gym and ask anyone, white belt to black belt, how their first ever BJJ class went. Everyone we’ve ever met who trains has a crazy beginners story that they tell with a smile on their face. Starting Jiu Jitsu is both a mental journey along with physical, and everyone around you understands that!