Hi! I'm an American female, too, but I'm a little shorter than you. About 10 years ago, I lived in Tokyo for a little over 3 years.
At first, I started out w/a student visa because I attended a language school. Then I taught English as an "Assistant Language Teacher" w/Borderlink. They require a BA though. Honestly, I wouldn't recommend them. Back then, they had this rule that you couldn't speak Japanese to the students, but the students all knew that I could speak it (JLPT N1). Also, the schools they work with tend to be out in the countryside. It took me about 3 hours each day to commute by train & on foot. I could've moved, but that's a huge task to do when you're alone in a foreign country. The pay wasn't awful (about $2200-$2400/mo.), but I spent most of it on transportation & rent. Rent was about $800/month in a tiny, old place about 20 min. by train from Shibuya.
I taught English & rotated between 2 middle schools (all grades) & 1 elementary school (5th & 6th grade). Most of the teachers were great. I even kept in touch w/some via Facebook or e-mail after I left Japan. However, there was one who just refused to call me "-sensei" or "Ms." in front of the students, so none of the kids would call me that, too. It's disrespectful, demeaning & just plain rude. Even students in America call their teachers "Mr./Ms." She also acted as if she was above everyone, even though she was actually newer to the school since she replaced one of the other English teachers. Near the very end, she finally called me "-sensei" or "Ms." during lessons, but that only happened after I told my supervisor. The kids were...kids. Don't expect them to be angels just because they're Japanese. Most were a lot of fun though & I bonded w/the ones who had similar interests.
About being overweight-- my case might be different from others since I actually gained weight after moving to Japan. It was mostly due to stress-eating & also because unhealthy food was cheaper & more filling than healthy alternatives. I started out slim for an American, but ended up average-chubby. Plus, the schools would let us bring home leftovers from lunch, so when you're low on cash, you're definitely not going to turn down free food. I was already overweight when I started teaching, but I wasn't obese. Most of my coworkers & students never mentioned anything about my weight. Some of the kids would say "oppai," but they were shushed quiet right away. There was one coworker (the same one in the paragraph above) who always acted haughty towards me. She might have looked down on me partially because of my weight (she was skinny even for a Japanese person), but she might have just thought I was inferior to her in general.
About making friends & dating-- I made most of my friends from interactions w/classmates or coworkers. I met some guys through Mixi, but be careful because a lot of them are just looking for one-night-stands (unless that's what you want). Even if you say you're just meeting up for karaoke, they'll try to take you to a hotel afterwards. I don't know how dating is in Japan nowadays, though.
Overall, it was mostly a positive experience. I'm glad I was able to do it, but I wouldn't want to do it again. I still go to Tokyo every year for vacation (pre-Covid) & can't wait till the pandemic is over. Best of luck to you!