I am in Canada so sizing is probably different.
At ~180 lbs in high school (13 stone) I was a lean size ~10. I'm 5'10 and I'm also very large framed (8" wrists if you want to go by that, but I have 8.3 lbs of bone in my body, and my husband who is the same height has 5.8 lbs, which is closer to average). So although I'm 6 inches taller than you, I think I understand where you're coming from. I wasn't inactive throughout life, but I never lifted weights or anything either. (Note: Cardio actually reduces lean mass, I've read recently)
At 15 stone, I believe I'd gone up to a size 14 (although I can't remember for certain). And by 21 stone - my max weight - I wasn't even sure. I've gotten new pants since then and I think they were size 18 from the plus sized section at Nordstrom. They're loosening up a little now.
If you're strong and you weigh that much and your measurements are good and you don't carry all your fat in your waist, then you can be pretty sure it's lean mass. If you want to be sure you can find out your body composition, either through pinch tests at a gym, a DXA scan, or other methods (the home scales aren't great but some places do a water weighing test that's good). That would tell you what your percentage body fat is, which is a far better indicator of health than BMI or measurements alone.
The nice thing is, you don't have to worry too much about losing some lean mass along with fat, if you have lots of it. In fact, it's the only way you're likely to get a long, lean look. So if you want that look I'd say, do cardio, burn as much as you can.
If your goal is to be healthy, you do NOT want to target a healthy BMI, because that doesn't work for people with a lot of lean mass. You want to target a healthy body fat percent. Think of it this way: Weightlifters might have very little fat on them, but if they stepped on a scale it would look like they were obese, for their height. You might not be a weightlifter but if you have more lean mass than average, you need to take that into account.
You will probably lose some lean mass while losing weight, and the faster you lose, the more lean mass you'll lose. If you lose more lean mass you will weigh less when you get into a healthy body composition range (because you'll have less fat and less lean mass)... but having less lean mass also means you don't get to eat as much, because lean mass is what burns calories all on its own while you're sleeping or being lazy. So really it's up to you as to where you set your goals.