If you want to start out the new year in an active way, the Matsudo City Nanakusa Marathon takes place in early January – and interestingly, it includes three different race distances which aren't actually marathon length. There are 2 kilometer, 5 kilometer, and 10 kilometer routes, with some of those options specifically designed for families, elementary-aged students, and junior high school aged students.
Please note that for the 10 kilometer route, the expectation is that the course needs to be finished by all competitors in 70 minutes. If participants aren't at the halfway point by the 35 minute mark, they will be asked to remove their bibs and clear the course.
Entries are open until October 31st, and can be submitted via fax (form here) or online here. Please note that the entry fees also differ depending on age and course length – the adults 10 kilometer event is the most expensive at 4500 JPY, but that includes a participation shirt which will be sent ahead of time so it can be worn at the race.