You sound similar to me. I've run 2:50-3:00 a few times on 6-8 weeks training.
I lose fitness pretty quickly. So with 6 months off I was in the worst shape of my life. Still wanted to go to Boston to enjoy the experience. 2 weeks out I "trained" for a week and then "tapered" for a week. In that 1 week of training I built up to a 12 miler at 8 flat pace. Hoped to run 8 flats for about 3:30 at Boston.
Didn't wear a watch, forgot to take off the time it took me to cross the start when looking at the 5k electronic board split....and so I went through the half in 7:0x pace. Definitely didn't think I was capable of that for a half. Those Boston downhills at the start just pulled my fat@ss along, though. Anyway, as expected I started walking about 15 in. Walk/Ran the last 12 and finished in the low 3:20s.
It was miserable, though. Absolutely miserable. If it hadn't been Boston, I wouldn't have finished. I thought about just walking in from 15 since I was on vacation. Given the finish time that would have put me at, I'm glad I didn't do that...