I usually picture Dragon Ball Z style training at this point and smile a little. I suppose you can get in shape by running, beating up a heavy bag, and sweating it out in a sauna. Or you could just start training in class again.
Don't let ego or pride get in the way of entering the dojo/gym. If you took six months, a year, or even longer off, there is no way you're coming back the same level you were at your prime. The same is true with any sport. But the best way to get back in shape and to start dusting off the cobwebs of your knowledge base is to start training in classes.
You don't have anything to prove. You don't need to submit your partners, execute a technique perfectly, or run someone off the mat with your head kicks. Accept that people who were once at the same level as you or lower are probably better or out ranking you now. Accept that you may have to sit out and catch your breath at times. Accept that you may not remember how to do that technique you were once super good at.
However, you have just done the difficult part. You took that first step, and now all it takes is time. Believe me, your skills and fitness will come back. I'm talking from experience. I had to take off while pregnant with Asher. Once I was finally back, I got a shoulder injury and was out again. Overall, I couldn't train normally for close to a year. But I can confidently say that my fitness and skills are finally back and getting better with every training session.
My advice for ex-martial artists looking to get back to training is to come to class! Put the ego aside and just start training again.