If you haven’t heard of the group 100 Miles, you’re probably not alone. Since its inception a little less than a year ago, the Brunswick-based environmental group has been keeping a low profile while doing its homework on myriad issues facing the 100-mile-long Georgia coast that inspired its name. “We’ll be a year old in November, and we’ve spent much of the first year figuring out how we fit in and how best to do our job,” said 100 Miles Executive Director Megan Desroisers.