In between training cycles

I just ran the Chicago Marathon this weekend with a time of 3h30, and I'm planning to sign up for the Carmel Indiana Marathon (end of April) with the goal of qualifying for Boston (aiming for closer to 3h20). The official training cycle for Carmel starts in mid-January, but I want to make sure I maintain the endurance and speed I've built from my recent training without burning out before the next cycle begins. What would you recommend I do over the next three months to stay in shape and prepare for marathon training without overdoing it?