Conventional wisdom is to take a break after the marathon and start fresh training cycle. Athletes who follow this approach typically need at least 12 weeks to be ready for another peak race performance. If you’re content with something other than a peak performance, you certainly don’t need to wait 12 weeks to race again. Just how long it takes to safely turn around from a marathon for another race varies greatly between individuals. As a first-time marathoner, you’ll just have to wait and see how quickly you recover. In any case, I would recommend that you train casually for the half and run it just for kicks.