dmm13,
Sounds like you and I are near carbon copies - running wise. I ran 2:45 this fall but raced a poor race. I set out way too fast, cruised through half at 1:17, started to die at about 19 or 20 miles, then mailed in the last couple of miles at about 8 minute pace. I felt I was ready to run 2:37ish, and still think I can when I get back into a full training routine. A running buddy here in town (he's about 50, but ran 2:25 and cracked 30 for 10k in his prime) is bound and determined I can break 2:30 based on my training pace (I tend not to run any slow runs, which may be my biggest problem). I think he's nuts, but I'm going to train with him through the winter and see what his sage advice does for my progress.
I'm nearly a Master's runner like yourself, creeping up on four-oh. Not sure if I have the guts (or the time) to crank my mileage up to where you're at, let alone where you're headed.
As far as lopping 18 minutes off your PB, that sounds like a challenge, but my four marathons (over ten years) have been 3:35, 3:15, 3:01 then 2:45. Mind you, going from 2:48 to 2:30 is probably like the jump from 3:35 all the way to 2:45. Having said that, only you know what you've got in you. If you work at it and stay healthy, you've got complete control (except over those factors you don't control, of course, like weather, etc etc).
Good luck with it!