Pick a new race. For many runners, that's the incentive they need to get back in the groove. Following a training schedule gives some structure to your running.
Join a charity team. If you trained on your own for your last race, you might find that the group motivation and running for a cause breathes new life into your training. Knowing that people are supporting your efforts also gives you motivation to stick to it.
Give yourself rewards. In addition to having a race to work toward, it helps to have rewards for reaching other goals. Buy yourself a new running outfit after you running regularly for a month, and then go for a massage or a pedicure every 2 or 3 months.
Do more cross training. You may feel a little burned out after running so much during marathon training. Mix things up and try some different activities, like biking or swimming, so you can build fitness without having to run all the time.