I'll start my post by saying I have yet to break 2:30. I've run 2:31 twice...BUT have since run 1:09:28 in the half.
That said, I totally agree with Link about running easy. For someone shooting for a 2:30 marathon, 6:00 pace for 12 miles is NOT easy. What I attribute much of my consistency to since moving up to the marathon is my ability to SLOW DOWN the majority of my runs. I was about your speed in high school, I was a 16:38 5k and 9:46 3200 guy. In high school and college, my training runs were never slower than 6:45 pace. Now all my easy days are 7:15+ pace...sometimes they even approach 8:00 pace, depending on how I feel. The only time I run hard (or moderate even) is during workouts or long runs. This allows me to string months together of consistent high mileage, being relatively injury free (some minor niggles, but everyone has those).
If you're just starting again...basically from scratch, that's all the more reason to run slower. And yes, it will take time to get your fitness back. Consistency is the key, don't worry about workouts for a while, just run.
I'm running Chicago this fall with the idea of a sub-2:30, though I think I could possibly run 2:26 on the right day. I'm trying out some Canova workouts leading up to the race, here's some Canova workouts if you're interested:
MARATHON WORKOUTS (LAST 6-8 WEEKS BEFORE TAPER)
Paces are based on a MP of 5:40/mile
Aerobic Power Workouts (one per week):
1) 10 x 800 w 1:30 jog @ 110-112% of MP (5:04-5:09/mile)
2) 6 x 1 mile w 2 min jog @ 108-109% of MP (5:12-5:15/mile)
3) 4 x 2 mile w 3 min jog @ 105-106% of MP (5:21-5:24/mile)
4) 3 x 3 mile w 4 min jog @ 103-104% of MP (5:27-5:30/mile)
5) 2 x 4 mile w 4:30 jog @ 102-103% of MP (5:30-5:33/mile)
5) 2 x 5 mile w 5 min jog @ 101-102% of MP (5:33-5:37/mile)
6) 6 mile continuous run @ 104-105% of MP (5:24-5:27/mile)
7) 8 mile progression run @ 100-106% of MP (5:21-5:40/mile)
Aerobic Endurance Workouts (one per week):
1) 18-22 miles steady run @ 90-95% of MP (5:58-6:18/mile)
2) 24-26 miles easy run @ 80-85% of MP (6:40-7:05/mile)
3) 18-22 miles progression run @ 85-100% of MP (5:40-6:40/mile)
4) 12-15 miles simulation run @ 100% of MP (5:40/mile)
5) 20 miles - last 5-10 miles @ 100% of MP (5:40/mile)
6) 20 miles incorporating 10 x 800 @ 103% of MP w 800 jog (5:30/mile)
7) 6-7 miles @ 85% + 6-7 miles @ 100% of MP (workout done AM and PM)