And 12 miles @ 6:00 as a workout for a 77 min half guy? Jesus...
That made me laught too. OP has plenty of speed to run a 2:45. A sub 1:18 is very good. My goal is also 2:45, and my half PR isn't that good (but I'm getting closer to that time).
I agree with others that you need to build up your long runs, and run them around 6:50 to 7:20 average. I typically do mine as kind of progression runs (similar to how I run a marathon), starting off slower, and doing the last three to six miles at 6:30 or so, giving me an average pace around 7 min miles or just under (without that much effort). (and the "close to MP" portion can get longer the closer you get to your taper).
I agree with doing tempo runs at 6 min pace, but for a 70-80 mile per week guy, 12 miles is way too long. That's like racing a half every week. Maxing out at 7 miles should be plenty.