The Hundred Mile Wlderness is one of my favorite hikes. I certainly wouldn't want to discourage you from doing the hike, but I would suggest that you recruit someone to hike with you. It is a challenging hike. On one of my hikes, I met a girl exactly your age in the middle portion of the hike. The hike was taking her longer than she planned. She was out of food and was running late enough that she knew her family was in a panic. I shared some of my food and let her use my satellite phone. There are a lot of people who can help you on the AT, but hiking alone involves more risk than hiking with a companion.
The suggested itinerary is a good one, if the weather is good. The most recent time I have done that hike, I had three days of steady rain. In the wet conditions, I found that I was unable to hike more than 10 miles a day.
As for your question about sneakers, footwear is a highly personal choice. What works for one person won't work for someone else. Personally, I wear Vibram FiveFinger Shoes and they work fine. In my opinion, the most important thing about footwear for that hike is to have something with a tread. You will walk on everything from slick rock to moss covered planks. Street sneakers don't have enough tread to provide traction when it gets slick.
If you would like to see pictures from the Hundred Mile Wilderness, I have some photo albums on my personal website --
http://www.davemcclung.com. It has been several years since I did that hike so the albums will be pretty far down on my album index.