I don't know that there's a one-size-fits-all formula for projecting improvement or for training toward an x:yy marathon. But one size that seems to fit most is higher mileage.
Here's my data point, rg, coming from a similar level and aiming for that same sub-3.
I started running in 2003 as a 35 year old. Prior to last fall I'd been stuck at 1:32 half marathon for the last couple years, generally averaging around 50-55 MPW.
Made modest gains from Daniels 10k training in spring 2006, then in late summer ramped the volume up into the 80s, including short (~4M) easy morning or evening runs most days. Moderate workouts largely from Hansons' marathon plan for the masses - a marathon pace run each week, another not too killer session of speed or tempo intervals. By October I was down to 1:23:47 for an 8+ minute half marathon PR.
I hadda be close to 3 hour marathon shape but sabotaged that race by going too hard on already worn legs the week after the half, messed up my legs and had to take a week off, ran a disappointing 3:11 at NYCM on stiff achy legs.
I'm convinced that at least for me, higher mileage, two runs most days, and relatively moderate, race-specific workouts that allow me to put in consistent miles without needing extensive recovery = a pretty good formula for the longer races.
You may find that with higher mileage you don't need the better part of a decade to get there, but of course we're all a little different. Best of luck.